For me, probably Glengarry Glen Ross (sp). So many great Pacino lines in that flick:
"I'm going to find out whose cousin you are and find a way to have your ass!"
"Who told you you could work with men?!"
"You stupid cunt! You fucking fairy!"
"You'd know it if you ever worked a day in your life: never open your mouth until you know what the shot is. You fucking child."
He does this slow burn right after Kevin Spacey blows his big sale. GREAT Pacino performance.
Others I liked him in:
Sea of Love
Looking for Richard
Any Given Sunday
and the very underappreciated Donnie Brasco.