We have to go deeper!
=Rate every character you can remember, based on their contributions to Season 2=
<Main>
Dany - Decent (due to a last minute improvement).
Jon - Bad. Waste of potential
Tyrion - Mostly good, great at a few moments.
Stannis/Melisandre/Davos - Started off poorly, but got very good soon enough.
Theon - Great (but his ending was only "decent"). Alfie needs some sort of award so his sister can't tease him anymore.
Bran - Decent.
Catelyn - Good (several great performances).
Arya - Good, but unfulfilling. Another waste of potential.
Cersei - Good + countless laughs = Great.
Sansa - Decent.
<Supporting>
Joffrey - Gold. Scene with Ros wasn't good, but everything else was great.
Robb - Boring (Bad).
Talisa - Boring but hot (So, a warmer kind of bad).
Roose Bolton - Decent I guess.
Jaime - Great for what he had.
Tywin - Great.
Lancel - Good.
Pod - Non-entity until the end. Came out slightly decent.
Pyromancer Hallyne - Great.
Brienne - Good, and getting better.
Balon - Decent.
Yara - Decent.
Dagmer - Good for his role, but I wish we could have had
Jaqen - A man was great.
Xaro Xhoan Daxos - Decent. Would have been better if he had been the richest man in Qarth.
Pyat Pree - Good first few episodes. Anticlimactic ending.
Quaithe - Forgettable.
Margaery - Good.
Loras - Decent but forgettable.
Bronn - Bronn/10.
Craster - Decent.
Jeor Mormont - Decent, for what time he had.
Qhorin - Just needed one more scene to be passable.
Jorah Mormont - Good, but not utilized.
The Mountain (new) - Bad, due to being completely untested.
Amory Lorch - Got the single biggest laugh of the season, so good enough.
Sam - Weak, but great ending obv.
Edd - Character hasn't been given a chance, but weak nonetheless.
Gendry - Only decent at best, since he kinda fell off the map.
Hot Pie - Hot. Pie.
Lommy - Hands not green enough.
Ros - Actually not so bad herself, but was present for a few problematic scenes.
Shae - Surprisingly good.
Ygritte - Good actress (and dat accent), but weak overall.
Varys - Great for what material he had.
Littlefinger - Decent at best. Get it together, Carcetti.
Pycelle - Good, for his screentime.
The Hound - Bad at first, good by the end of the season.
Doreah - Disappointing ending.
Irri - Will be missed. It is known.
Tommen - Needs more kittens.
Drogo Vision - Really good.
Baby Rhaego Vision - Good hair growth.
Osha - Good.
Rickon - Taught those walnuts an important lesson.
Hodor - Hodor.
<We hardly knew ye>
Renly - Great. A huge improvement over Season 1. Turns out he
was right after all.
Luwin - No complaints.
Yoren - Ditto.
Rodrik - Ditto pt.2.
Spice King - Will be missed. Corrected spelling for generations to come.
<Animal Pals>
Summer - Barely there.
Shaggydog - Same.
Grey Wind - Prettay, prettay good. Best In Show of this season's direwolves.
Ghost - Bad dog. Left his master to the whims of the wildlings. Probably out eating pinecones or something else that dumb dogs do.
Dany's dragons - Drogon stood out, but the others need to tighten up their game.
That chicken Jaqen kicked - Solid stuntwork.
Zombie Horse - Decent screen presence.
An overall average of Season 2: Good.
Weakest links: Jon and Arya (due to missed potential). Everyone else is tough to seriously complain about, but those two are some big problems.
In a surprising outcome, Theon and Cersei were my MVPs. Stannis/Melisandre were the best new additions. Tyrion, it can be said, had the "easiest" role this season but Dinklage was great by the end. All of Joffrey's screentime (aside for the Ros scene) was amazing. House Lannister was definitely the best-handled part of this season.
Of course, just my personal opinions.