may as well throw in my cents regarding the team i used ingame, currently training to take on gym 7
Growlithe: Middle Tier
Availability: Early on, after the first Gym.
Stats: Respectable all-around, no real glaring issues.
Typing: Fire isn't necessarily bad, though it goes through a rough spot late-game where there isn't all that much weak to it.
Movepool: Gets Fire Fang early on, but has to wait until level 45 Flare Blitz to get a better physical Fire attack. At the same time it gets Flamethrower and Heat Wave and both are more than usable. What makes it so great is the lategame movelist, though, with both Crunch and Outrage (!!!) granting excellent type coverage. The biggest problem is you have to hold off on evolving it to get any of it's good attacks, and getting a Fire Stone might be a problem.
Major Battles: As mentioned it falls off late game; Burgh is easy, but Clay and (presumably) Marlon give it problems. The rest aren't necessarily easy nor hard.
Additional Comments: Really, Growlithe is just sort of in the middle. Good but not amazing stats, a movepool that starts out fine but doesn't pick up until lategame, neutral against most of the gyms.
Roselia: Middle Tier
Availability: A tiny bit later then I'd like but pre-gym 4 certainly isn't the worst timing.
Stats: A good special attacker by all means, though a tiny bit fragile.
Typing: Grass/Poison is unoriginal as hell but it works fairly well, with a relative lack of Psychic types to eat Roselia alive.
Movepool: Roselia's biggest problem is that it only learns Grass attacks. That said, the second Gym's TM is Venoshock, so by the time you get it you already have access to a good Poison attack for it. Said Grass attacks tend to be surprisingly good, going from a somewhat-early Giga Drain to Petal Dance to Energy Ball. It also gains Leech Seed and Synthesis, and can learn Cut for the two or so small trees in Unova, so only having two attacks isn't necessarily horrible.
Major Battles: Roselia shows up just in time to wall Elesa's (non-Emolga) team, after which Clay's gym is a free exp buffet. It falls off for Skyla and Drayden, but stands well against Marlon, especially if you kept Giga Drain.
Additional Comments: Would probably be High Tier if it came just a little bit earlier, didn't need a stone to evolve and learned any other non-Grass moves. Poison Sting as it's only non-TM Poison STAB is kind of painful.
Trapinch: Low/Middle Tier
Availability: After the third Gym, which isn't too bad, especially since the fourth Gym is Electric.
Stats: As Trapinch, it has acceptable Attack and enough bulk to not die to everything. Once it's fully evolved, Flygon is more than acceptable, with a balanced yet high stat spread.
Typing: Ground/Dragon is a pretty good type in-game, as long as you're smart enough to keep it away from Ice types. Which there aren't actually a lot of since Brycen got kicked off the gym roster.
Movepool: Early on it gets Rock Slide and Bulldoze, which is enough to carry it to the midgame. When it gets there, though, it's stuck with only Dig and, even later, Crunch. On the bright side it gets Dragonbreath and Dragon Tail as soon as it evolves, and Dragon Claw naturally, but if you want to get Earthquake you have to hold off until a whopping level 55, long after it would normally have fully evolved, and then use Heart Scales for Dragon Claw. Unless you're masochistic enough to keep using the pitifully-slow Trapinch you aren't getting Earthquake until post-game.
Major Battles: Trapinch shows up just in time for Elesa, but despite having a type advantage over Clay's Excadril he simply isn't bulky enough to handle it. Only one of Skyla's team members is easily taken by Rock Slide, by the time you get to Drayden he'll have to handle a mutual type weakness and most likely won't have anything better than the decreased-priority Dragon Tail, and he just doesn't have much to take Marlon with.
Additional Comments: Trapinch is honestly a bizarre Pokemon. When you catch him, 100 base attack is devastating, but he's let down by his speed. The problem is, ironically, when he evolves: Vibrava loses the power of Trapinch, and although it gains enough speed to keep up, it simply doesn't have enough bulk to handle the midgame. Things get better when he fully evolves, but Trapinch and Flygon have the same attack, meaning unless Trapinch's speed is your biggest problem you might be better off leaving him unevolved until you get Earthquake (probably around the 8th gym level-wise), and do you really want a NFE on your team endgame?
Mareep: Middle Tier
Availability: One of the earliest, before the first Gym even.
Stats: A bit on a low side all-around, and the Mareep line is uncharacteristically slow for Electric-types, but he still manages to get by.
Typing: Pure Electric has ups and downs. The ups being STAB Discharge, the downs being dying to anything that even smells like dirt.
Movepool: Offensively, Mareep's options are generally restricted to Electric attacks. Getting Electro Ball naturally midway through the game is a boon, and Discharge isn't bad at all. Once you're nearing the mid-lategame, Power Gem appears to give more type diversity, and even later Signal Beam shows up as well. When it comes to support, a VERY early Thunder Wave is delicious, and being one of the few Pokemon to get the +3 defense Cotton Guard is a definite advantage. The biggest fault is the pure Electric bend, and getting Thunderpunch on evolution to Ampharos is just silly. Remember when Ampharos could use Thunderpunch effectively with his stubby little arms? In GSC?
Major Battles: For the most part Mareep does well. Cheren, Roxie and Burgh are all relatively neutral, Clay and Elesa are a brick wall, Skyla is utterly devastated, Drayden is problematic and Marlon is delicious fish. One thing to remember is that Thunder Wave can give Mareep an advantage over most neutral opponents.
Additional Comments: Really, there are two big problems keeping Ampharos in middle tier. One, it's a bit slow. Two, it isn't Magnemite. Seriously, Magnemite just outclasses Mareep in most ways. Still, Amphy is just cool as hell.
Heracross: Low/Middle Tier (Black only)
Availability: Like Roselia, you can catch Heracross before the fourth gym. Not painfully late, not comfortably early.
Stats: Being an unevolving Pokemon, Heracross comes packaged with excellent stats, including a beastly Attack and enough speed to use it.
Typing: Bug/Fighting, although giving Heracross a critical Flying weakness, isn't actually that bad. Or rather, it wouldn't be, if...
Movepool: Megahorn at level 46. That alone sums up everything wrong with Heracross. Granted, the rest of it's movepool is actually good. Brick Break and Close Combat at low-medium levels is excellent, and it even comes with Aerial Ace. Night Slash is available as well if you don't mind coughing up a Heart Scale at the PWT in Driftveil. That said, the biggest draw of Heracross is that it's a Bug/Fighting type, and it's capable of learning exactly two Bug-type attacks before the endgame: Megahorn at level 46 (around the seventh gym) and Struggle Bug via TM (a pathetically weak special attack). That's it. You get the X-Scissor TM before gym 5, but Heracross can't use the bloody thing. Without any STAB, a Bug typing is purely a downside, and when it comes to simply beating things to death with Fighting STAB Timburr comes earlier, has an arguably better movepool, lacks a 4x Flying weakness and does just as well.
Major Battles: With the exception of Skyla, Heracross is pretty much neutral to everything. Not being resisted by Drayden is nice, and the type advantage over Clay's Excadril is welcome.
Additional Comments: Seriously, if this thing could use the X-Scissor TM it would easily be high tier. As it stands, there's no reason to use Heracross over Timburr unless you REALLY need a kickass Bug STAB and don't mind waiting for it.
Ducklett: High Tier
Availability: Mid game, just a few feet from the fifth gym.
Stats: Actually really good. Fast with enough attack and special attack to hit hard from both sides of the spectrum.
Typing: Yes, it's Water/Flying. Yes, it has a 4x Electric weakness. But you get it AFTER the fourth Gym, and Electric Pokemon aren't especially common. Water and Flying are both useful STABs.
Movepool: As soon as you catch it it has Bubblebeam and Aerial Ace, which alone is enough to use (remember, it's attack and special attack are the same!). Just a few levels later it gets Air Slash and Roost, and in the lategame it picks up Brave Bird and Hurricane. Combine that with Surf compatibility and Swanna has everything it needs to work excellently. The problem is that even with TMs it'll never get out of the Water/Flying STAB rut, but neither of those are bad STABs anyway.
Major Battles: You get it just before the Ground gym, where it excels, and it's neutral to every other gym going forward.
Additional Comments: Seriously, this thing gets way too much hate for it's typing. If you don't mind waiting a little bit for it, Swanna is definitely a top-tier ingame Pokemon, Electric weakness be damned. It's just a shame it has to live with the reputation Pelipper gave Water/Flying Pokemon.