My top 10:
1. Sion/Taric/Annie/Ashe - Raw, long stuns. Best there is, no questions asked..
2. Malzahar/Warwick - Suppression. No casting and no motion. Gotta love it.
3. Cho/LeBlanc/Kassadin/Garren - Silences. Next best thing to a stun/suppress, IMO. Stops them from casting.
4. Rammus/Shen - Taunt. Can't cast, and you can walk them to where they belong. GG.
5. Poppy/Alistar/Gragas/Tristana - Knockbacks kick ass. They pull people away from the scrum, interrupt and can separate people for a good beating.
6. Swain/Veiger - Delayed stun. Limited use, but very effective if they land.
7. Morgana/Lux/Ryze - Snares are BA, but lower on the totem. people can still do what they want, they're just stuck in place.
8. Janna/Cho/Blitz/Maokai/Malph/Xin - Knock-ups are cool and all, but they only interrupt, and only have a flash of disable.
9. Leblanc/Kennen/Panth/Maokai/Etc - Situational stuns. They're good when you need a quick escape route, but are markedly less effective than hard CC.
10. Slows. All slows. Great CC when paired with any of the others, but on the whole aren't exceptionally useful.
My choices for best:
Sion, Taric, Rammus, Gragas, Lux, Annie, Ashe, Cho, Veiger, Garren