I always think of De Niro as my favourite, he's in so many of my favourite films. I love Pacino too though: despite not being as consistent as De Niro, when Pacino is good he's awesome!

Some of my fave De Niro films:

Sleepers
Heat
Casino
A Bronx Tale
Cape Fear
Goodfellas
The Untouchables
Angel Heart
The Mission
Once Upon a Time in America
Raging Bull
The Deer Hunter
1900
Taxi Driver
The Godfather: Part II
Mean Streets