This is my rating of the kits, at my level:
0 = Worst kit ever
1000 = More OP than OP
Beast: 432
Warrior: 673
Sniper: 355
Wizard: 292
Samurai: 847
Beast and Warrior aren't that OP, since everybody uses them and it's usually a fair fight.
Sniper isn't that effective at any range since people do move quite a lot and only a few people use it. Many people using Beast and Warrior just target non-Beast-Warrior-kit players.
I think Warrior is slightly better than Beast, since Protection II Diamond Armour doesn't do much to reduce damage but the Sharpness II sword can inflict a lot of damage in seconds.
Wizard kit is useless at long range and is only effective if you throw the Instant Damage potions into a group of people at 1 second intervals, or if you throw Swiftness potions and drink Strength potions and charge at a lone player.. The Poison potions do nothing to the enemy. Players who use Wizard kit often get chased by a large group of Beast kit players and Warrior kit players mostly of their leather armour and the fact they are considered "noobs" by them because of their armour. As a result they die because they can't fight off over 10 players, can they?
Samurai kit, in my opinion, is quite OP because of the speed effect, because this allows you to forever run around in speedy circles around the opponent who hasn't a clue where to hit. The knockback with Samurai is amazing as well, allowing time for you to escape. The only bad thing about Samurai is that its, armour sucks, just Protection II iron chestplate and boots, or even, in wings maps, Protection II iron boots. Samurai could do with better armour - turn the current armour to Protection III and add an iron helmet, also enchanted.
This is just my opinion. Feel free to compliment me or criticize me, I won't mind.