All right, here's my GBU Singles team. It's still very much in the trial stage, so suggestions are welcome!
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Bisharp @Dark Gem
Adamant / Defiant
252 HP / 252 Atk
-Sucker Punch
-Swords Dance
-Substitute
-Iron Head
This guy is the main physical attacker of my team. A Dark-Gem boosted Sucker Punch can OHKO virtually anything that doesn't resist it. Substitute on a predicted switch allows Bisharp to block status and grab a SD or two.
Reuniclus @Toxic Orb
Bold / Magic Guard
252 HP / 252 Def
-Calm Mind
-Psychic
-Focus Blast
-Recover
Wall time. Maximum physical bulkiness allows Reuniclus to win against specially bulky T-Tar one-on-one. I have seen this guy tear through entire teams after a few CMs.
Gastrodon @Leftovers
Sassy / Storm Drain
252 HP / 252 SpD
-Scald
-Earth Power
-Toxic
-Recover
Pretty self-explanatory here, Toxic beats bulky Water-types that don't carry Grass attacks. He also completely walls Rotom-W, who would otherwise cause problems for the team.
Froslass @Ice Gem
Timid / Snow Cloak
252 SpA / 252 Spe
-Ice Beam
-Protect
-Disable
-Thunder Wave
This is the team's oddball, and my trump card. Snow Cloak is a nasty surprise for a Hail team. With a base speed of 110 and a Timid nature, Froslass outspeeds virtually anything that isn't Scarfed or Jolly Thundurus. Ice gem guarantees a OHKO on bulky DW Dragonite and can seriously mess up Ferrothorn. Froslass is extremely frail, so I use a variation of the SubDisable strategy to block slower opponents' most threatening moves. Once Disabled, an opponent will often be forced to switch, allowing Froslass to paralyze the switch-in and subsequently Disable it. Froslass can usually survive a Thunderbolt from Jolly Thundurus, Paralyze it, and proceed to use either Disable or Ice Beam. Disabling can also open up gaps for my pokes to switch in when they normally wouldn't be able to, more on that in the next paragraph.
Scizor @Choice Band
Adamant / Technician
252 HP / 252 Atk
-Bullet Punch
-U-Turn
-Superpower
-Pursuit
This is a fairly standard set, but it can still wreck unprepared teams. Scizor has good synergy with Froslass, who can allow it to switch into Fire-types (especially Chandelure) and Pursuit them on the switch.
Rotom-W @Choice Specs
Timid / Levitate
252 SpA / 252 Spe
-Volt Switch
-Hydro Pump
-HP Fire
-Trick
Rotom-W is a good team player because it can evade Earthquakes aimed at Bisharp. It also plays well with Scizor by absorbing Fire-type attacks, harassing bulky-Water types, and playing VoltTurn mindgames with the opponent. HP Fire can nail Ferrorthorn switch-ins and Trick messes up walls and boosting sweepers.
My team still has huge problems with Sun teams, particularly Ninetales. I'm thinking about switching out either Reuniclus or Bisharp for a specially bulky Tyranitar - this would also help dealing with the problematic Thundurus.
