I'd say Gary Oldman for the reasons stated already. Also Robin Williams (mostly), Tom Hanks, Keifer Sutherland, Kenneth Brannagh, and Rufus Sewell. Special mention goes to Bruce Campbell.
Worst for me would be Tom Cruise, Ben Stiller, Ryan Reynolds, Vince Vaughn, both Luke Wilson and Owne Wilson, Chris Tucker, and Christian Bale.
However one actor that seemingly no one has mentioned, and most people don't even know is John Noble. While his part in Lord of the Rings movies was rather minor and mostly forgettable, his current roll(s) on Fringe have consistently impressed me. Often I'm literally brought to tears by the sheer amount of sorrow he conveys, even as he makes me laugh. I don't think that anyone in the cast could be easily replaced, and its a shame that there are only 3 episodes left.
Sure some of the earlier plot threads went nowhere, and some are needlessly convoluted and never really detailed, but it was still a great show. I feel that after the most recent episode they might just wrap it all up neatly, or at least neater than it has been in those final 3.
As for actresses I sadly can't think of any standout performances other than A.J. Cook and Paget Brewster from Criminal Minds, the women on Once Upon a Time, Rachael McAdams, Jodie Foster, Julia Roberts, Olivia Wilde on a few episodes of House M.D., or Amy Acker from Angel and Dollhouse. I know Charlize Theron is a good actress, but I can't name any movie of hers I have ever seen. I can't stand Scarlet Johansson, and Kristen Stewart can join Paris Hilton, Lindsey Lohan, and Kim Kardashian on the list of women that need to exit our collective consciousness.
Just because I like Tina Fey, Emma Stone, Kat Dennings, Alyson Hannigan, Drew Barrymore, Jennifer Connelly, Emily Paige, Liv Tyler, Rachel Leigh Cook, Joely Fisher, and Jennifer Garner to name a few doesn't mean that they are great actors. It means that they are beautiful, smart, and funny and typically they aren't offered or encouraged to take on really demanding roles. Its just the nature of the business. They all have an image to maintain whether they want to or not. You usually don't get those kinds of roles until you are over 50 and typically British it seems. Whereas Nic Cage can literally take on any role for better or worse because he rivals Michael Jackson on having the most out there public persona in media. He may not be the best actor, many would contend he is the worst, but he certainly isn't niche.
Worst actresses for me would be Lindsey Lohan, Julia Stiles, Sarah Michelle Geller, Halle Berry, and Kristen Stewart.
Admittedly I don't watch movies often.