I'm relatively new to the world of competitive battling, however I've tried my hand at a team, so please, give me feedback and assistance as I am new to this so I may make some rookie mistakes.
Lucario @ Lucarionite
Trait: Steadfast / Adaptability (Mega Forme)
EVs: 252 Spe / 252 Atk / 4 Def
Nature: Jolly
~ Hone Claws
~ High Jump Kick
~ Iron Tail
~ Stone Edge
I've been running M-Lucario as a lead on my Showdown team for the past few weeks, and it hasn't failed me yet. If you aren't scared of an adaptability High Jump Kick, you really should be. Hone Claws makes this move hit 97% of the time and increases Iron Tail's accuracy as well, allowing Lucario to check fairy types as well as offer it excellent STAB and neutral coverage. Stone Edge deals with pesky fire types and the ever present (and ever annoying) Talonflame. If Lucario is given the chance to set up, it has some serious sweeping potentials, Adaptability really is scary.
Greninja @ Life Orb
Trait: Protean
EVs: 252 Spe / 252 SpA / 4 SpD
Nature: Modest
~ Hydro Pump
~ Blizzard
~ Dark Pulse
~ Grass Knot
Greninja has a great ability and alongside useable stats and a diverse movepool, it can type-shift it's way into being immune or resistant to most types. It can be used to set-up, to spread status, as a physical attacker, or a special attacker, it has no limitations and this makes it extremely difficult to counter. I've run a modest nature in order to maximize it's offensive prowess and help it get those 2HKO's. Hydro Pump/Blizzard can easily be switched for Surf/Ice Beam respectively, however the extra BP really helps Greninja out. Dark Pulse offers better coverage than Extrasensory and I've thus opted for that. Grass Knot helps decimate things that would otherwise wall it (eg: Swampert / Gastrodon etc). It does however face a large threat in the form of Ferrothorn.
Terrakion @ Choice Band
Trait: Justified
EVs: 252 Spe / 252 Atk / 4 Def
Nature: Jolly
~ Close Combat
~ Stone Edge
~ Earthquake
~ Quick Attack
A fairly standard Terrakion set. I've used Terrakion in Gen 5 and it is yet to fail me, whether it be banded, scarfed or just with a life orb, it's amazing typing and STAB combo hits hard. This thing is sent in to clean up the leftovers or make an initial chip in the opposition. Close Combat/Stone Edge/Earthquake are standard moves for this legend, Quick Attack helps it counter faster threats who may be low enough on health to get a banded KO. Clearly it needs to be as fast as possible and thus I'm running a Jolly nature.
Azumarill @ Splash Plate
Trait: Huge Power
EVs: 252 Atk / 252 HP / 4 Def
Nature: Adamant
~ Belly Drum
~ Aqua Jet
~ Play Rough
~ Ice Punch
With Belly Drum and Aqua Jet legal, this relatively unused Poke gets hurled up through the tiers and proceeds to scare the shazam out of pokemon trainers everwhere. Belly Drum makes its attack soar and when combined with a priority STAB move such as Aqua Jet, Azumarill is sure to leave a dent in anything it hits. It's new Fairy typing also grants it great resistances however it also leaves it painfully vulnerable to Scizor's bullet punch, which hits it hard. It also loses half it's health when setting up which can be an inconvenience.
Noivern @ Choice Specs
Trait: Frisk
EVs: 252 SpA / 252 Spe / 4 SpD
Nature: Modest
~ Draco Meteor
~ Focus Blast
~ Hurricane
~ Flamethrower
I ran this set in game and Noivern acted as a fantastic revenge killer. However it does have the over-used Dragon/Flying typing, it's given a fantastic STAB in the form of Hurricane. Whilst the accuracy of Hurricane and Focus Blast is only 70% (Not to mention it tends to miss at the most crucial moments), they're both invaluable coverage moves for Noivern. It's blazing speed already outspeeds most and a Specs and modest nature is applied for maximum offensive potential. It works great to leave a dent in things or finish off weakened threats with powerful STAB moves. Flamethrower is better for coverage than Boomburst, and whilst it is 140BP it hits nothing super-effectively and thus I've left it off this set.
Espeon @ Leftovers
Trait: Magic Bounce
EVs: 252 SpD / 252 Spe / 4 HP
Nature: Calm
~ Calm Mind
~ Psychic
~ Shadow Ball
~ Toxic
Espeon is really a test for me at the moment, it switches in and counters anything that attempts to use status or hazards and it also possesses decent bulk. Calm Mind makes it great at being specially defensive as well as increasing it's SpA letting it hit hard with 2 great coverage moves. Toxic hurts anything it's moves can't cover and leftover's helps it be a tad more bulky. I'm not sure of it's place on my team at the moment, as I am considering running Starmie as a spinner instead.
~
End-Notes:
Do note this team is a WIP and any comments or suggestions will be taken into account and are greatly appreciated.
I've tried to keep it even in terms of Special and Physical attackers (I have 3 of each atm). However I would generally run a Salamence on my team as a moxie sweeper.

Lucario @ Lucarionite
Trait: Steadfast / Adaptability (Mega Forme)
EVs: 252 Spe / 252 Atk / 4 Def
Nature: Jolly
~ Hone Claws
~ High Jump Kick
~ Iron Tail
~ Stone Edge
I've been running M-Lucario as a lead on my Showdown team for the past few weeks, and it hasn't failed me yet. If you aren't scared of an adaptability High Jump Kick, you really should be. Hone Claws makes this move hit 97% of the time and increases Iron Tail's accuracy as well, allowing Lucario to check fairy types as well as offer it excellent STAB and neutral coverage. Stone Edge deals with pesky fire types and the ever present (and ever annoying) Talonflame. If Lucario is given the chance to set up, it has some serious sweeping potentials, Adaptability really is scary.

Greninja @ Life Orb
Trait: Protean
EVs: 252 Spe / 252 SpA / 4 SpD
Nature: Modest
~ Hydro Pump
~ Blizzard
~ Dark Pulse
~ Grass Knot
Greninja has a great ability and alongside useable stats and a diverse movepool, it can type-shift it's way into being immune or resistant to most types. It can be used to set-up, to spread status, as a physical attacker, or a special attacker, it has no limitations and this makes it extremely difficult to counter. I've run a modest nature in order to maximize it's offensive prowess and help it get those 2HKO's. Hydro Pump/Blizzard can easily be switched for Surf/Ice Beam respectively, however the extra BP really helps Greninja out. Dark Pulse offers better coverage than Extrasensory and I've thus opted for that. Grass Knot helps decimate things that would otherwise wall it (eg: Swampert / Gastrodon etc). It does however face a large threat in the form of Ferrothorn.

Terrakion @ Choice Band
Trait: Justified
EVs: 252 Spe / 252 Atk / 4 Def
Nature: Jolly
~ Close Combat
~ Stone Edge
~ Earthquake
~ Quick Attack
A fairly standard Terrakion set. I've used Terrakion in Gen 5 and it is yet to fail me, whether it be banded, scarfed or just with a life orb, it's amazing typing and STAB combo hits hard. This thing is sent in to clean up the leftovers or make an initial chip in the opposition. Close Combat/Stone Edge/Earthquake are standard moves for this legend, Quick Attack helps it counter faster threats who may be low enough on health to get a banded KO. Clearly it needs to be as fast as possible and thus I'm running a Jolly nature.

Azumarill @ Splash Plate
Trait: Huge Power
EVs: 252 Atk / 252 HP / 4 Def
Nature: Adamant
~ Belly Drum
~ Aqua Jet
~ Play Rough
~ Ice Punch
With Belly Drum and Aqua Jet legal, this relatively unused Poke gets hurled up through the tiers and proceeds to scare the shazam out of pokemon trainers everwhere. Belly Drum makes its attack soar and when combined with a priority STAB move such as Aqua Jet, Azumarill is sure to leave a dent in anything it hits. It's new Fairy typing also grants it great resistances however it also leaves it painfully vulnerable to Scizor's bullet punch, which hits it hard. It also loses half it's health when setting up which can be an inconvenience.

Noivern @ Choice Specs
Trait: Frisk
EVs: 252 SpA / 252 Spe / 4 SpD
Nature: Modest
~ Draco Meteor
~ Focus Blast
~ Hurricane
~ Flamethrower
I ran this set in game and Noivern acted as a fantastic revenge killer. However it does have the over-used Dragon/Flying typing, it's given a fantastic STAB in the form of Hurricane. Whilst the accuracy of Hurricane and Focus Blast is only 70% (Not to mention it tends to miss at the most crucial moments), they're both invaluable coverage moves for Noivern. It's blazing speed already outspeeds most and a Specs and modest nature is applied for maximum offensive potential. It works great to leave a dent in things or finish off weakened threats with powerful STAB moves. Flamethrower is better for coverage than Boomburst, and whilst it is 140BP it hits nothing super-effectively and thus I've left it off this set.

Espeon @ Leftovers
Trait: Magic Bounce
EVs: 252 SpD / 252 Spe / 4 HP
Nature: Calm
~ Calm Mind
~ Psychic
~ Shadow Ball
~ Toxic
Espeon is really a test for me at the moment, it switches in and counters anything that attempts to use status or hazards and it also possesses decent bulk. Calm Mind makes it great at being specially defensive as well as increasing it's SpA letting it hit hard with 2 great coverage moves. Toxic hurts anything it's moves can't cover and leftover's helps it be a tad more bulky. I'm not sure of it's place on my team at the moment, as I am considering running Starmie as a spinner instead.
~
End-Notes:
Do note this team is a WIP and any comments or suggestions will be taken into account and are greatly appreciated.
I've tried to keep it even in terms of Special and Physical attackers (I have 3 of each atm). However I would generally run a Salamence on my team as a moxie sweeper.