Batman - Christian Bale is hard to beat but I wonder what a young Gregory Peck or Burt Lancaster might have done with the role. As an 'Elseworlds', I also can see David Boreanaz having done well if he'd beaten Bale to the part in 2005.
Robin - Edward Furlong, circa 1994 - 95.
Alfred - David Niven (he was linked to the role when a Batman pic was first talked about in the aftermath of Superman The Movie), Peter O'Toole.
The Joker - I'd like to see Chris Eccleston in the role. For a vintage one, what about Conrad Veidt, whose role in The Man Who Laughs inspired the character's look. Jake Busey also looks right for the part and is probably sufficiently crazy, given his parental genes!
The Penguin - Modern day Paul Giamatti, retro Peter Lorre. I know he's not quite the fit for the role, but I think he would have made an interesting creepy Penguin.
Two-Face - James Cagney, Errol Flynn or Ed Norton
Ra's al Ghul - Christopher Lee or Omar Sharif
Talia Al Ghul - Rachel Weisz
Comm Gordon - Another vote for Oldman but a younger Stacy Keach would also have been a good fit for the role.
The Riddler - I still want to see him played by someone smart, creepy and smug, like David Hyde-Pierce, Jeffrey Combs or Christoph Waltz.
Catwoman - Marilyn Monroe, Eliza Dushku.
Lucius Fox - I really like Morgan Freeman in the role, even if his Morgan Freeman shtick is a bit familiar by now.
The Scarecrow - A younger James Cromwell
Batgirl - a younger Dina Meyer (who played the role in Birds of Prey) or Alicia DeWitt.
My dream Batman cast wouldn't feature Mr Freeze or Poison Ivy, so I don't have to cast them!