If Kubo had actually planned passed the Evil-Aizen reveal and didn't just start making shit up without putting any thought into and have him finally beaten by a deus ex machina then I would pick Aizen, buuuuut he did, so Tobi all the way.
Aizen! I like Tobi, but eh. Aizen is just cool and badass, and I love all the different "sides" of him XD I liked Tobi before we found he was supposed to Madara, and I'm getting ticked that we haven't seen his, and if it isn't revealed soon!! And it better be a major plot-twist.
Ehhhhm... I'm going to have to vote for Aizen, based solely on the fact that his plans don't sound quite as much like the ramblings of a psycho ward escapee as MADARA's do. If it was a looks contest, Past Madara would have won out over past Aizen, but present Aizen kicks present Madara's ass (I really hope his hair isn't as short as it looks.)