Answer: Televisionary: Y'know, Pam, I'm going to take the cowardly way out and drop the "est" part of the question because, frankly, in order to figure out who is the biggest of all time I'd have to not only add up the box-office takes of blockbusters starring Hanks or Willis, I'd also have to add up the totals of guys like Clint Eastwood, who got his first big break on Rawhide and has had a longer career. So allow me to talk in generalities, please.
Certainly, Hanks and Willis claim their share of the biggest domestic box-office earners and Hanks is bigger, money-wise. The former, who started out on Bosom Buddies, has starred in such huge hits as Forrest Gump, Toy Story and its sequel (voice), Cast Away, Saving Private Ryan, etc. But the latter, who first burst into the public consciousness on Moonlighting, is no slouch either, boasting successes like The Sixth Sense, Armageddon, the Die Hard movies and Pulp Fiction.
This is where I get into the kind of trouble that generates tons of mail. I'm sure people will dig up all sorts of details on who appeared in features before making it big on TV and whose cumulative totals internationally make them the biggest. But that's why I'm not going into detail. Again, we're talking generalities.
Certainly, Eastwood's up there since his films (the Dirty Harry movies, The Bridges of Madison County, In the Line of Fire, Unforgiven), while not topping the list of blockbusters, include a number of very successful movies and probably muscle him in through sheer quantity.
And a partial (very partial) list of the rest would certainly also include those who may have appeared in movies first but really launched their careers on TV and those who made it in the music world first before jumping to TV and then to film. Standouts include Leonardo DiCaprio (the TV version of Parenthood, Growing Pains), Jim Carrey (In Living Color), Tim Allen (Home Improvement), Robin Williams (Mork & Mindy); Denzel Washington (St. Elsewhere); Sandra Bullock (the TV version of Working Girl), Cher (Sonny and Cher), Will Smith (The Fresh Prince of Bel Aire), George Clooney (Roseanne, ER), Danny DeVito (Taxi); Geena Davis (Buffalo Bill); Billy Crystal (Soap); Michael Douglas (The Streets of San Francisco); Johnny Depp (21 Jump Street); Steve Martin (The Smothers Brothers Comedy Hour); John Travolta (Welcome Back, Kotter); Brad Pitt (Glory Days) and Warren Beatty (The Many Loves of Dobie Gillis).
And just getting into the soaps world (Alec Baldwin, Robert De Niro, Kevin Bacon, Dustin Hoffman, Meg Ryan, Susan Sarandon, Martin Sheen, Demi Moore) and Saturday Night Live (Eddie Murphy, John Belushi, Adam Sandler, Dan Aykroyd, Chevy Chase) would make the list way too long, so I'll stop now. (And please, don't bury me with those I missed I know.)