It's really hard to pick just one or two, but here are a couple that I really like.
*Leonardo DiCaprio (specifically his newer movies like Inception, Shutter Island and The Departed)
*Christian Bale (I really like him in all the movies I've seen him in, he really knows how to play different roles)
*Keanu Reeves (I know I'll probably be one of the few, but I really like most of his movies. I'm not saying that it's him "acting good", it's more his whole style that I like)
*Edward Norton (He's really one of those actors who I feel manages to make an impact on a movie with his performance. American History X, Fight Club and The Italian Job just to name a few of his movies that I really like)
*Guy Pearce (for Memento alone really, his performance in that movie is brilliant)
*Simon Pegg & Nick Frost (for their brilliant performances in Shaun of the Dead and Hot Fuzz. Paul was a huge disappointment, but seeing as Edgar Wright didn't direct that one, it was kinda expected. Have not watched Spaced yet, but it's on my list)
*Richard Coyle (Okay so I've only seen him in Coupling, but his performance as Jeff is one of the most brilliant I've come across in a sitcom. If you haven't seen Coupling - DO EET.)
*Brad Pitt (I didn't like him for a while, but when I managed to see Inglorious Basterds, Fight Club and Se7en within a few weeks, I got a whole new respect for him. Those 3 movies are just brilliant, but I kinda find a lot of his other work to be hit or miss)
When it comes to actress, I'll go with these:
*Natalie Portman (Not only is she quite possibly the hottest woman on earth, but she is also a fantastic actress. Her performance in Black Swan was IMO her best in a movie that I've seen, but she has always been good in the movies I've seen with her)
*Sigourney Weaver (Mostly for the Alien-movies. She's really a great main-character in those movies, and being a huge fan of that franchise, it's hard to not like her. Solid in other movies as well)