Overall, I'd say I gravitate towards people who put a lot of effort into their craft. Maybe it's because, ultimately, I'd love to be an actor, but seeing the amount of work that some of these guys put into their transformations, both physical and mental, just impresses the hell out of me, so, as far as guys like that go, I'd say:
-Christian Bale
-Daniel Day Lewis
...and, most recently, I've found Matthew McConaughey to be an immense talent, and someone who has really grown over the years.
Then, there are the icons. Clint Eastwood is probably one of the single greatest assets the entertainment industry has ever seen. Just the idea that he was such a legendary actor, and, yet, he also has tremendous talent as both a director and, even, a composer.
Cary Grant, Gregory Peck, Marlon Brando, Jack Nicholson etc.
Other actors I'm partial to are Tom Hanks, Harrison Ford, and Michael Keaton. Al Pacino's great, as is Deniro. I respect Stallone and Schwarzenegger because they play to their strengths and don't let anything get them down. Schwarzenegger, in particular. I think Bill Burr said it best, when he was joking about how people try to undermine Schwarzenegger with the maid thing. He said that "Schwarzenegger should've been loading trucks in Austria," but, instead, he went on to become Mr. Olympia, become a box office superstar (who could barely speak English), marry a Kennedy, and become Governor of California. Affair or no affair, I absolutely give the guy props for his accomplishments and have always found him to be very entertaining.
I'll be flat out honest, here, Batman introduced me to a few of my favorite actors. Prior to Batman Begins, Christian Bale was an unknown to me, but, then, I was still just a boy when that film started production. He was electric on screen and I was just drawn to anything he was in. I hate horror films, for the most part, yet I loved American Psycho, his collaborations with David O. Russell are just fantastic, and I've even loved his work in Out of The Furnace and Rescue Dawn, and I couldn't believe my eyes the first time I saw "Empire of the Sun;" that he was so talented, even then, at such a young age.
Keaton was similar, he's not as much of a character actor as Bale, but he's still incredibly talented. Those Batman films were the bait, but, once I bit, I'll watch pretty much anything with that guy in it. He's a fantastic comedian, as well. Beetlejuice was one of the only ones that I may have known him from at the same time I knew him as Batman, but the guy is just so charismatic. Night Shift, Johnny Dangerously, The Other Guys; hell, as panned as it was, Multiplicity is one of my favorite comedies of all time.
Television-wise, I think Bryan Cranston's the name that pops into everyone's minds. Jeff Bridges is great in everything I've ever seen him in. Same goes for John Goodman.
Actresses: I think Audrey Hepburn is just a joy to watch in anything. Marion Cotillard's great in a lot of her stuff.
I don't know. There have been so many talented individuals to pass through the film industry that it's really hard to pin down a "best of" list. These are just some thoughts off the top of my head, and I just know that I'm missing several of my favorites.