I finished my game of W2 on Challenge Mode (Which is no more challenging btw). So here was my team:
Snivy - Mid Tier
Availability: Starter
Stats: Great Speed (Outspeeds anything you need to), Respectable Defenses, but low Offenses. Considering the move Coil, I like this stat distribution because he can boost his Atk and Def (and Acc) before the opponent attempts a physical move. Brace for impact, then swing right back. Plus, his speed means he can revenge kill.
Typing: Just grass. It comes with a few bad weaknesses (notably fire and flying) but it helps for neutral coverage. Also the home of Leech Seed and Giga Drain.
Movepool: Not huge, but enough. For a physical set, he has Coil (Great boosting move) and two strong, useful moves Leaf Blade and Return, that offer for great coverage. As for the last moveslot, He can opt for Leech Seed, Giga Drain, Dragon Tail, or Aqua Tail. Leech Seed helps ALOT early game and any of the rest can help late game. With Giga Drain, he can set up against the water type gym and gain hp back with giga drain.
Major Battles: He never really shines in any gym besides the water gym, but he can still put a good fight in a majority of the gyms (except maybe Flying). In earlier gyms, he tanked through a couple pokemon with leech seed (and other pokemon's help of course); and later gyms we went offensive.
Additional Comments: Not many people like serperior because he lacks raw power. I nominated Serperior for Medium because he became tedius to use late-game. He was always lagging behind and I chose to level him for the sake of leveling. Regardless, if you're willing to set up with coil each time, he can level pretty fast.
Growlithe - High Tier
Availability: Virbank Complex (Inside)
Stats: All around Great. 90/110/80/100/80/95. Nothing too low, but none are very high either. 110 atk was great though.
Typing: Fire. Great offensively, not too great defensively. He's not frail like Darmanitan though, so he has longevity when certain fights call for it.
Movepool: A lot of Physical moves to work with. Flare Blitz for starters, then other moves include Crunch, Outrage, Wild Charge, and ExtremeSpeed. On the special side, he has Flamethrower/Fire Blast and also Dragon Pulse. All of these are Arcanine's useful moves, which his non-fire options help distinguish him from Darmanitan. I ended up going with Flare Blitz / Crunch / Outrage / Fire Blast (When facing Physical tanks)
Major Battles: He didn't perform any miracles as Growlithe, though he destroyed Burgh's pokemon just fine. My Growlithe was adamant with almost perfect Atk EV's, so he could hold his own until evolution. As an arcanine, shit got real when He solo'ed Drayden's gym with outrage. With a couple levels behind, he outspeed and OHKO'ed or 2HKO'ed each one (I did have to cure his confusion though).
Also a groundbreaking note. My arcanine defeated Iris's Haxorus. It tanked an earthquake with a few hp and KO'ed after 2 outrages (2nd was a crit). Darmanitan can't do that.
Additional Comments: Raising Growlithe isn't too much fun, but he can hit pretty hard with flame wheel in the 20's, flamethrower in the 30's, and Crunch/Outrage in the 40's. Then he gets Flare Blitz which means evolution time. Also, playing on Challenge mode allows him to evolve earlier in the storyline (In my case, right before encountering Cobalion). Lastly, he can use the eviolite to make him feel less like a baby.
Zubat- mid-High Tier
Availability: Castelia Sewers
Stats: Just Damn fast, Respectable Attack, Respectable Bulk, Low SpA, but usable with Nasty Plot if you can breed (also good special moves too)
Typing: Poison / Flying. The flying is all you need, but it helps to quad-resist fighting, grass, and bug. The flying comes with a rock and electric weakness which hurts though.
Movepool: ACROBATICS 110 bp w/o an item. That is basically the only offense he needs. He also has access to Cross Poison (heartscale), X-Scissor, Bite, Fly (key HM), confuse ray, and screech. On the special side, you have Dark Pulse, Giga Drain, Sludge Bomb, and Heat Wave. Acrobatics alone renders X-scissor and Bite near-useless because if it's insane power with Stab (165bp > 160 bp or 120bp). U-Turn is also a great move if you want to go into the PWT. I went with Cross-Poison / Acrobatics / Fly / Screech (To make threats like hydreigon or haxorus frail for my teammates).
Major Battles: Upon catching Zubat, I made immediate use of him against Burgh's gym. At Elesa's gym, he was already a Crobat; but he has way too much trouble in that gym (Plus Acrobatics wasn't learned yet). After getting acrobatics, he pulled his weight just fine against the gyms that called for him.
Additional Comments: I didn't use him much in the E4, but only because I didn't need him. I attempted to use him against Marshal and Iris, but I made the BIGGEST choke in my life by giving him an Expert Belt. I went for the SE hits, but completely forgot about the way Acrobatics functions, and didn't realize it until he died to Hydreigon (I got to screech him though!). I still nominate him for a high tier though. He hits very hard with Acrobatics and can outpace any pokemon in the game, considering EV's. I guarantee I would've defeated haxorus with this.
Staryu - Top Tier
Availability: Undella Town (Rippling Water), actually pretty easy to encounter and catch. You'll have a water stone to evolve it immediately. He comes rather late, but honestly, you can deal with it.
Stats: Well laid out. Very fast, useful Sp. Atk, and okay defenses.
Typing: Water and Psychic, which is a great combination. Water for great neutral stab, and Psychic for destroying Marshall.
Movepool: Everything you need and more. Upon catching it, you'll have access to surf and Psychic is in the route north of it. When getting to Lacunosa town (bordering the route you find psychic in), you have access to Thunder/Blizzard. Though if not, then you can opt for Thunderwave (purchasable TM) and/or Confuse Ray (heartscale). Late in the game, you'll be able to use Ice Beam and Thunderbolt. In the end, I ran Surf / Psychic / T-Bolt / Ice Beam, and hooked on an expert belt.
Major Battles: He could battle anytime, anywhere honestly. I didn't use him so much in gym battles, but he had some fine exposure in the E4. He had some key kills such as the fighting type crew, Iris' Hydreigon, and Outspeeding and 1HKO'ing a choice Scarf Chandelure.
Additional Comments: The reason I gave Staryu the top tier is because of his insane potential with a good EV spread. With the ability to outspeed everything, good SpA, good offensive typing, a wide movepool, and an expert belt, he can surely wreak havoc on any team. *Also a note: Black two gives you access to Power Anklet and Power Lens pre-E4, so EV training for Speed/SpA is EASY. Staryu and Litwick deserve a top tier for such potential to abuse that.
Scraggy- Top Tier
Availability: Route 4 (before Burgh)
Stats: Pretty slow, but great defenses, and good atk. Speed EV's seriously help in the long run.
Typing: Dark/Fighting, the best you can wish for Offensively and Defensively at the same time.
Movepool: The Dual-Stab is all I used in game: Hi Jump Kick / Crunch / Brick Break / Strength as HM filler. Ice Punch is a viable tutor move though.
Major Battles: I didn't selfishly use him to abuse any gyms, but he carried a shitload during the E4. I benched him against the Ghost trainer because of CoFagigrous' Mummy, but I let him Solo the Dark and Psychic type trainer (I even outspeed and OHKO'ed Metagross 4 levels higher than scrafty). And for fun, I OHKO'ed White Kyurem with HJK. Fun Times
Additional Comments: Scrafty is probably the most reliable pokemon you could get. He is difficult to take down, and he can even build momentum with Moxie, which it's just a simple sweep. Also, his base speed at 58 is nothing to cry about because a good majority of pokemon can still be outspeed by scrafty when you invest a good load of Speed EV's (which isn't difficult at all).
I was ready to nominate Metang too, but I literally used him once, and I just died to Iris's Haxorus. Honestly, I didn't need him, but I'm sure he would've wrecked the Elite 4. (188atk at lv 53 for a clue)