Picardo is definitely one of the best actors ever to play a regular Star Trek role. He's equally convincing during moments of pure hilarity as well as during very tense, serious scenes. "Tinker Tenor" is a perfect example of this. In one scene, he's showboating around the bridge and blowing up the Borg in his daydreams, and then in the next, he's hiding in Sickbay, thoroughly humiliated over the fact that his friends have been witness to his private fantasies. And in both scenes, he's completely believable.
Another great example of Picardo's acting range is the episode "Life Line". He plays the Doctor and Lewis Zimmerman as two distinctly different people: similar, but with enough subtle differences in personality that you easily forget that you're watching ONE actor play TWO people. Even their voices are slightly different; Zimmerman's voice is more dry and informal, and more "organic", while the Doc has a more precise way of talking, but also has more warmth in his voice.
Yeah, Picardo's got serious acting chops. He's my favorite male Trek actor, along with Patrick Stewart, Brent Spiner, Rene Auberjonois, and Andrew Robinson.