1995 Arkansas, 2005 Vermont and 2010 Butler are my "big 3" in terms of gut-wrenching heartbreak.
Was too young to remember Keith Smart, thankfully. And Richmond is a very vague memory for me as I was 8 and didn't really care yet.
The first tourney games that I felt emotionally invested were the 1994 season with Hawaii, Wisconsin-GB and Missouri.
My big four are Iowa in 1980, IU in 1987, Illinois in 1989 and OSU in 2012. Reason being those were all very good SU teams with a shot at a title. Btw notice a trend with these losses? Some of those other losses were painful but those teams weren't going very far anyway.