He is a very good dramatic actor. Not the best - he doesn't have the range of many actors I could name - but for that limited thing he does, he is very good. He is completely convincing and that's all any actor can hope for.
As a comic actor, he is not just average but weak. At times he can actually make me feel embarrassed for him with his attempts to be funny. That's not good for an actor. Which wouldn't matter except he keeps trying to do comic roles - so it has to be factored into the rating. Rocky and Bullwinkle? Really?
But the drama is still most important and overall I don't have him in the very top tier. He is in the tier below that. Pacino is right around the same spot but for slightly different reasons.
Excuse me, I forgot this thread is being done in dopey forumspeak of overstated superlatives where opinion is fact. Let me re-summarize:
Deniro is, like, easily NOT in the top tier. Bar none.
As always, my opinions are easily the most correct. Bar none.