Man, I came here to mention Jeffrey Combs!
I can't say the same about Marc Alaimo no matter how much I loved Gul Dukat, simply because I haven't seen him in anything else!
But also +1 to Max von Sydow, mentioned upthread also. And Alan Rickman.
And Sarah Shahi, because there aren't enough actresses being mentioned, and everything I've seen her in, Life, Person of Interest, etc. she's been awesome.
Geoff Stultz (The Finder) and Damien Lewis (Life) and Donal Logue (Gotham), as well.