That's tough. I can't decide right now, even though I'm leaning towards my personal favorite, the oft-underrated John Cusack. Sean Penn would be up there too. If you stopped the voting at say, 1996, Pacino and DeNiro would both be my top choices, but they're movies over the last decade have really collectively blew.
I would be ok with either Pacino or DeNiro but my own personal favorite in a long time would have to be Ed Norton.
Honestly, all 4 of those guys basically play the same character time and again. Gary Oldman would definately be my pick. He is just a chameleon who disappears into every role.
Given that time frame, I think it could easily be argued that Merryl Streep is by far the greatest actor of the last 30 years.
DeNiro is on record of saying that Streep was his favorite actress to work with - a true professional. My pick is DeNiro by a hair over Pacino.