“Going in Style”, with George Burns.
I hated the end (the actual treasure scenes) due to the “yeahhhh… rigggght” factors, but I loved the fact it was actually filmed at Independence Hall and other places where they got to film portions you’ll never see as a tourist.
When I was at the National Archives recently the seemingly rather put-upon tour guide/security guard kept saying on a loop “…and no more questions about National Treasure, it was a movie, it was fiction, it couldn’t happen, one of the agreements in letting them film here was that they couldn’t say anything remotely real about how we store the documents… the one correct thing in the movie is that you are currently standing in Washington, D.C.”.
Another couple of movies based on books by Donald E. Westlake:
Bank Shot
Jimmy the Kid
neither are great cinema but both are fun. Jimmy the Kid is a kidnapping that goes horribly wrong. A kidnapping is a heist of a person, isn’t it?
By the way, I love the Hot Rock and would put it up against any other comic caper gone wrong movie.
Probably one of my 5 favorite movies. I meant that it was “sort of” a heist movie.
Just remembered The Score with Robert De Niro. Not great, but worth checking out.
The Ladykillers. I’ve only seen the Tom Hanks version, which was quite fun, but I hear the original Alec Guinness version is better.
I second Sneakers. Great movie if you like high-tech stuff.
I saw this film as a kid, and I bought it on DVD in a fit of nostalgia. There was some good stuff in it, but I don’t think it’s aged very well.
Ocean’s 11 (both versions since I think the Rat Pack veersion was great.)
The very film I was coming in here to mention. You sir, have fine taste.
No love for Who’s Minding the Mint? Featuring an actual Rat-Packer!
[size=1]And Gilligan and Cpl. Klinger*
I’ll second this, with the addendum that Zero Mostel is very possibly the funniest man who ever lived.
Another in the “not great cinema, but a really fun movie” category: After the Sunset.
Gambit. More of a romantic comedy / heist, plus it’s two heists in one: you see it played out as planned, and then as it actually comes off.
Sweet, thanks. I see my movie nights being filled for a long time. I have seen a couple of the movies suggested, not many though. Thanks all, and keep 'em coming 
Incidentally, AMC’s running a strip of the first seven episodes on Saturday, February 18. Just what I’ve been waiting for to jump in on “Hustle.”
I haven’t seen anyone mention, The Pink Panther. The real one, of course, with Peter Sellers, David Nivin, Robert Wagner, etc.
John Wayne’s ode to the type is The War Wagon
Also:
Sam Whiskey takes the Heist movie and puts it into the old west to rob the Denver Mint. Shallow but OK. Starred Burt Reynolds, Clint Walker, and Ossie Davis.
How to Steal a Million with Peter O’Toole and Audrey Hepburn. It throws in romance and it makes a good film.
Don’t Raise the Bridge. Lower the Water Jerry Lewis - Really, really weak.
River, actually, not Water.
Some of these may rely on a loose definition of “heist” or be a bit too comedic for the OP, but, in my books, they rank as good heist films nonetheless…
Sexy Beast
Palookaville
Bottle Rocket
The Newton Boys
Croupier
A Fish Called Wanda
Resevoir Dogs
Confidence
Lock, Stock & 2 Smoking Barrels