Spoiler Thread: Avengers Infinity War
Fantastic villain. I will put Thanos close to Ledger's Joker (yeah, I went there). As stated earlier in this thread, Thanos has his own beliefs and is actually trying to make the universe a better place. He is willing to sacrifice those he loves for the greater good (the greater good). He just isn't some jerk on a power trip.
Similarly, the Joker isn't a jerk on a power trip. Rather, he is someone with something to prove (that everyone is just like him--they just need a bad day). Give into chaos, and your life will be better (why be a hamster on a wheel?). If anything, the Joker doesn't want power. There is a belief system for him (as warped as it may be) that he sticks to--just like Thanos did.
I almost wish that they would end this version of the Marvel Universe with this movie, because I think any movie that follows will lessen it. They will find a way to bring everyone back and it will make the sacrifices less in my opinion. It will be harder to stay vested in characters that can die and be brought back to life. One of the reasons I think DC went too soon into the Death of Superman arc.
They sure could have used Ant Man on Titan. Throw a disk on the gauntlet to make it bigger, when if falls off, another disk to make it smaller--then take it and run away.