It's a tough call, but I'm going to have to go with the obvious choice for Best James Remar Line Ever. It's from the 1979 film The Warriors, where James plays a gang member named Ajax who utters this witty bon mot to a baseball bat wielding foe:
"I'll shove that bat up your ass and turn you into a popsicle."
I'm not certain, but that may be my favorite line from any movie ever. The only problem with it is that it's very context-specific; it's not a line that you can quote in public with a reasonable chance of a) it being appropriate, and b) anyone knowing that you are indeed quoting James Remar in the first place. It's not like saying "Say hello to my little friend!" when you pick up a chicken skewer at a buffet table or "May the force be with you" before somebody does karaoke. These are movie quotes that people might actually know and that could be situationally appropriate.
But unless you're at a Little League game, there are very few scenarios in which you could say "I'll shove that bat up your ass and turn you into a popsicle" and have it make sense. Come to think of it, while it might make sense to say that at a Little League baseball game, it should probably be avoided.
Anyway, it's still the greatest James Remar line of all time, even if it's not that handy.