TNG is a tie between Frakes and Sirtis. I can enjoy the charms of Frakting but he has several stock mannerisms that he just turns to over and over and over. Watch how he cocks his head back and slightly narrows one eye, that's Ironic Mode: Activated. And he'll grin and jerk his head slightly when he's being That Roguish Riker. Once you spot them, you can more or less call them in advance just before he does them.
You can see this same sort of thing with ANY actor if you see enough of their work. Leonard Nimoy did an episode of The Untouchables ("Takeover") in 1962, and you can spot a lot of pre-Spock mannerisms. He's also with William Shatner in The Man From U.N.C.L.E.'s "The Project Strigas Affair" (shot around the same time as "The Cage" pilot judging from Nimoy's haircut), and you can recognize little bits of business from both of them.
For actors, those little recurring mannerisms of their own personality are tools they use in creating a character.