Kenneth Branaugh, as Gilderoy Lockhart in Harry Potter & the Chamber of Secrets. Of course, it was the type of role that HAD to be over-played to be effective. 
Oh, and a second vote for Withnail & I, both for Richard E. Grant (Withnail) and Richard Griffiths (Uncle Monty). Griffiths seems particularly gifted at playing a boorish old fop…he was Uncle Dursley in the Harry Potter movies, too.
Dennis Hopper in pretty much everything.
Christopher Walken - At Close Range
Val Kilmer - Tombstone
Nathan Lane - The Birdcage
“Listen up ya primitive screwheads”
This far and no mention of Bruce Campbell in the Evil Dead trilogy? Army of Darkness is especially enjoyable for me. He’s such an arrogant ass in that one.
“This is my BOOMSTICK!”
I’ll mention Carole Lombard and John Barrymore in Twentieth Century, not to mention many of their other roles. And I’ll agree with Crunchy Frog on Bruce Campbell’s performances in those movies. In fact, I was watching Army of Darkness as Crunchy posted that. I enjoy that pretty bad movie immensely.
Another vote for Peter O’Toole. Magnificent as Henry II in “The Lion in Winter” and “Becket”.
I still say R. Lee Ermey was born to play that role.
You can’t bring up over-the-top performances without mentioning Rod Steiger, especially in his later years. At the end, he was pure Cure 81.
Sid Haig in “Jason of Star Command,” an old Saturday morning kid’s TV show. He played the villian, and when Jason thwarted him, you could practically see smoke coming out of his ears.
Since I did so well nominating Peter O’Toole, I’m going to nominate Rosiland Russell – specifically her role as Mame Dennis in Auntie Mame (1958) – as the most enjoyable scene-chewing actress. She had everything – sophistication, screwball physicality, good lucks, devastating quips and high-energy. You see in long stretches of her other movies but none so brilliantly as AM.