Other than TGBU, the best western ever made, I've seen him in the Two Jakes with Jack Nicholson, an okay sequel to my favorite private eye movie Chinatown, and he had a bit part in Mystic River (great to see him paired up again with Clint, even if Clint was only behind the camera).
Tuco was my favorite character in TGBU, of course you were rooting for Blondie, but Tuco was such a cool character. He was an evil bad piece of dung, but when he fought with his brother, you felt bad for him. Eli Wallach was an excellent actor in that movie. Angel Eyes, Tuco and Blondie. I think Mikkel Willumson named his knives after these characters. I loved John Waynes movies, but for me the gunslinger was Clint Eastwood. Eli Wallach helped cement that with his performance in TGBU.
RIP sir, you were a helluva actor.