For some of the best that come to mind I think of Johnny Depp playing Jack Sparrow as Mick Jagger. Just about all the choices Heath Ledger made for depicting the joker (speech pattern, walk, etc.). Val Kilmer's speech pattern for Doc Holiday in Tombstone.
For some of the worst I think of Don Cheadles British accent in the ocean 11 movies. Damon Wayan's performance in Bamboozled as an uptight uppity brother (this may be more miscasting then poor choices though). Initially I hated Daniel Craig's Southern Drawl in Knives Out but now on repeat I just see it as camp.