Serperior
Leech Seed/Growth/Giga Drain/Leaf Blade
Obviously my starter, and was pretty good throughout the whole game. Nice in the beginning, it 2HKO to 3HKO'd most trainer's Pokemon and could take quite a hard hit too. His low attacking stats are a total non-issue. He hits just as hard as any other Pokemon.
It began to slow down a bit before evolution, but after that it was as powerful as ever with Leaf Tornado. It was brilliant for quite a while and didn't really slow down before evolving again, because it had the 5th Gym to train in. After evolving into Serperior, it was better than ever, but that's where it started to go downhill. As I finally got together a team of six members, it started falling behind the others and was quite overshadowed by them. There were plenty of opportunities in which I could have sent in Serperior to save the day, but instead I ended up sending out other Pokemon because they just get the job done better. To sum him up, I'd say he's good at what he does, but he's limited and outclassed to an extent.
Upper Mid Tier
Zebstrika
Wild Charge/Thunder Wave/Flame Charge/Thunderbolt
As odd as it sounds, this was a great team member. Got it very early on Route 3, already with Shock Wave as an attack. Hit surprisingly hard and was useful against the majority of the early-game.
It learned Flame Charge in Pinwheel Forest, which was very useful for all the Sewaddle and Venipede that kept appearing. Soon before it evolved, it learned Spark, which did massive damage compared to Shock Wave. I gave it an Eviolite while it was still a Blitzle to help it survive more attacks; it worked very well and it became a solid Pokemon.
After evolving, things just got better. It hit insanely hard at that point in the game. It learned Discharge later, which was actually on par with Spark in terms of power and was a very reliable move to use. I went and got the TM for Thunderbolt as soon as I got Surf, which of course I taught to this guy. It was an absolute killing machine, no lie. It was fun to use, helped me through a lot of the game and kept the momentum going when things got rough (if it weren't for him I'd have easily lost to both N and Ghetsis). He was one of those Pokemon you could send into a sticky situation and have it almost always come out on top.
Top Tier
Sigilyph
Fly/Psychic/Shadow Ball/Air Slash
There was only one issue with this Pokemon; it made everything too easy. Seriously, it just tore through the game when I got it; there's barely any need for me to explain after all the praise it's got already. I've got no complaints, I could have quite easily solo'd the game with it.
Top Tier
Galvantula
Volt Change/Charge Beam/Thunder/Signal Beam
A real pain to level when I first got it after the 5th Gym, but as soon as it evolved, it was a pretty nice Pokemon. It had a nifty typing for the upcoming Gym. The real deal came when I taught it Thunder, where it was clean OHKO'ing quite a lot of the game. I would report much more on it, but I only sent it into favourable situations because Zebstrika was my main Electric-type throughout the game, but I just had to get this little guy as well. I couldn't resist.
High Tier
Ferrothorn
Curse/Ingrain/Gyro Ball/Power Whip
An odd choice for a team member, and the best way to describe its performance is just that; odd. It was ridiculously hard to level up at first, barely damaging anything with its moves and being unable to run from anything due to its Speed (it still can't). It took me about an hour in total to grind it to evolution. After that it started to catch up quite quickly. Its STABs did a lot of damage, and while Gyro Ball sounds great off of such a low base Speed, a lot of Pokemon at that stage are still quite slow, and so it's not as effective unless you use Curse. So if you run into something slow that resists Power Whip, you either have to switch out or start Cursing. On the other hand, if you do run into a fortunate Speed advantage, you can deal absolutely massive damage. It's odd because Ferrothorn relies not on type advantage alone, but the opponent's Speed and coverage moves. It can survive an onslaught from many Pokemon, it just might not always be able to do much back, and if it runs into a Pokemon it could easily kill that has a Fire move, its Speed causes it to switch out as well. In general, it performs very well where it is able to, it just can't switch into any old situation and promise you a quick defeat like many other Pokemon can.
Keeping it in against Ghetsis's Sazandora and Gyro Balling it to death after a Focus Miss was hilarious, though. Made me love it so much more.
Mid Tier
Chandelure
Confuse Ray/Will-O-Wisp/Fire Blast/Shadow Ball
First of all, the first two moves are of no use whatsoever.
When I first caught it, it was able to defeat most of Celestic Tower's Pokemon, with the aid of about 50 Super Potions and 20 Revives. The situation was always kill or be killed for Litwick. After many Audinos and one evolution, it became a lot better and was winning far more than it was losing, having become much more reliable. Evolving it into its final form as early as possible is highly recommended; Chandelure is an absolute beast. It wrecks everything with its dual STAB, and it is the thing I often found myself sending into anything that was threatening my team in order to take care of it. It does come kind of late and is a pain at first, but the pay-off is well worth it. As of post-N's Castle, it has 160 Special Attack while the rest of my team have attacking stats lingering between 90 and 130.
Upper Mid Tier