Welcome to Fluther. Lurve.
James Cagney was terrific as a gangster (he was terrific in other roles too).
Bogart was tough and beleivable. Edward G. Robinson was scary. Robert Deniro was great, as was Al Pacino. Harvey Keitel is wonderful too.
But the best ever? The most believable, ruthless gangster? Michael Madsen in Reservoir Dogs. That guy is piss your pants scary, and totally reminds me of my brother.
Cagney was originally a song and dance man; he greased his hair with vasoline. Here he is in 1933. My mother is on the far left in the dark shorts; starlets had more meat on them then.