Top roles for League of Legends

For a variety of reasons, your teammates should make sure that they're always familiar with at least 6 champions in any given position. That sounds absurd but it's relatively easy to imagine a situation in which the team first picking bans three champions in one position in an attempt to completely screw a lane on the other team. If they pick first and pick that position, it means your team got their 5th choice. If your team ALSO banned a player in that position (which is easy enough to do) then you're getting your 6th choice. So who are the best options in each role? I'll provide the best options based on the most recent tournament

AD Carry: Ezreal, Graves, Corki, Vayne, Ashe, Caitlyn
Jungle (old, unclear on current viability): Cho'Gath, Amumu, Dr. Mundo, Maokai, Skarner
Jungle (new, projection without evidence): Warwick, Nunu, Amumu, Fiddlesticks, Lee Sin, Olaf
Support: Zyra, Lulu, Blitzcrank, Sona, Leona, Nami
Mid: Diana, Orianna, Twisted Fate, Katarina, Karthus, Elise
Top: Lee Sin, Rengar, Olaf, Shen, Malphite, Yorick

Those are based on the "traditional" roles. However, there's some other roles that should be considered that don't QUITE fit the meta right now but COULD come back.

Assassin: It's unclear whether these are best in the middle or coming out of the jungle (or in a support capacity?) but there could still be something here.
LeBlanc, Akali, Diana, Ahri, Shaco, Evelynn, Kassadin

AD Jungler: There's some logic in having some sort of pseudo double jungler where one jungler takes the red side and one takes the blue side. The red side jungler is the "kill" jungler and the blue side is the "support" jungler. Under this setup, the red side would be a more AD focused jungler. Examples are:
Yorick, Zed, Warwick, Udyr, Olaf, Nocturne

AP Jungler: These are expected to jump out and support. Examples are:
Nunu, Cho'Gath, Skarner, Alistar, Rammus

AP Support: There's a few support characters that really help AP characters (generally through reducing MR and /or providing some CC)
Morgana, Soraka, Fiddlesticks, Ryze

AP Counters: There's a class of characters that are very much designed to kill AP characters or otherwise neutralize them
Galio, LeBlanc, Kassadin, Talon

AD melee carries: For a variety of reasons, the current meta strongly discourages these but that doesn't mean they don't have a place. Note, the characters with good sustain are typically considered "bruisers" and found in the top lane (Yorick for example). These characters are ones that need a support character to excel
Darius, Fiora, Tryndamere, Riven

AD counters: There aren't too many of these but a few characters have innate abilities to counter traditional AD carries, at least early on.
Fizz, Poppy, Sion

Split Pushers: Some characters make it into current lineups for OTHER reasons and they end up split pushing frequently but few teams FOCUS on split pushing in the current meta
Shen, Heimerdinger, Sivir, Ziggs, Nasus

So those are the traditional roles that get filled and the rough list of people who fulfill them. If you're asked to play a role, make sure you fully understand at least 3 of them enough to work around bans and preemptive picks. You should endeavor to learn 6 of them and i'd also recommend learning at least one other position inside and out (ideally you'd know all positions). Lastly, it's worth playing at least a little bit as the champions that typically counter your favorite champions. If you love Katarina, learn to play as Diana (for example). Most people are under the perception that Katarina is almost hard countered by Diana. Knowing how to play against a Diana can be very useful. Sometimes just knowing a champion's inherent weaknesses can allow you to be aggressive at times when you wouldn't normally have been.