I'm skipping the least favorites cuz there really aren't any abilities I dislike... My list would just be the countless abilities that are next to worthless (not even counting stuff like truant; at least that makes a difference vs.... hyper cutter... damp... etc)
Favorites:
10- Pixilate: I rather like its sister abilities as well, but Fairy type is best type and it sounds digital-ish too so we're going with this one.
9- Drizzle: More rain pls. give 5th gen.
8- Unaware: I like this one solely because I think it would be my ability. I'm sorry, did you say something? I forgot to listen.
7- Trace: Perhaps a common theme on this list, I like abilities that allow you to make plays, so to speak. Switching into copy someone's flash fire or magic guard is one my favorite things to do.
6- Regenerator: I'm naturally inclined to stall. Being able to take an advantage out of switching is very appealing to me. I also really like the idea of regenerator cores, although there's less effective type synergy there than I would like.
5- Contrary: Similar to Pixilate in that it opens up some various strategies, while also creating a space to laugh at stuff like intimidate and sticky web... I'm a fan. ...even if Serperior is my bane.
4- Shadow Tag: Come. Sit. Stay a while. Die slowly, with nothing you can do about it. <3
3- Protean: Similar to Trace. I like that there are outplays to be made here, but with high risk for failed predictions.
2- Magic Bounce: Much like the ladder. When someone stays ahead of the game they can get by without hazard control if they have this, which I find really neat. If you mess up, it's on your prediction.
1- Prankster: I adore prankster because, even if just through gimmick, it seems like everything that gets it finds some unique way to use it. Prankster Encore? Check. Swagplay? Check. Priority Perishtrap? Check. +3 Spa Baton Pass, repeat Hazard Phasing, turning mediocre Pokemon into a lovely status wall, check, check, check. It's just wonderful. I love it.