Who is better?

Kayle is fun because she is viable in every role, and can build more or less any items. Anivia is a late-game monster, but her laning phase and mana problems are spectacularly painful. Soraka is super obnoxious to lane against, and has a lot of teamfight presence with her super-spammable heal and annoying silence. Nami is pretty good at everything, honestly. A good all-around support. Honestly, it really depends on what you like to do. They all have their niches.

goes like this in mid lane anivia kale raka nami in sup nami raka kale anivia

http://33.media.tumblr.com/tumblr_m3lmc138uc1rrs54to1_250.gif

Yorick

At what?

At farming minions? Anivia. CCing? Nami. Healing? Soraka? AoEing enemies down with holy fire? Kayle.

Who's the most fun? I prefer Anivia out of the four you gave me. Pushes lanes easy and keeps assassins at towers, relieves pressure elsewhere, safety net, icewall. Kayle never stood out as fun for me, and Soraka can have a yawnfest lane.

Who's the strongest? Meh, depends on if fed. I think Kayle classifies as a hypercarry, or close to one.

Soraka in low-mid elo is pretty much a free win, and is irritating even at high levels.

You have to focus her, but it is not rewarding to do so.