I think a sharingan would be badass. Plus i think it would be awesome to have the taijutsu of rock lee or mito guy. Also with the speed of Naruto. It's a little realistic, plus I'd love to charge in and spinning kick someone in the face. Plus, I think the walking on water and walls is awesome.
Although if I had to pick my favorite tragic power with faults, I like the idea of being able to take other peoples' luck away for yourself or giving your own luck to others.
The Rinnegan because it can use all forms of ninjutsu, possibly even Susano'o, Kekkei Genkai, Sharingan techniques, etc. Who knows? We'll find out what the Rinnegan's true power is later.