Tough one. In no particular order...
Cancun for Spring Break in 96 (I was a college tours rep and spent 6 weeks there)
Mardi Gras 2000 (Spent a week with 12 buddies, we drove down from Buffalo in a Winnebago and stayed right on Bourbon)
Summer in Europe in 97 (highlights included 2.5 weeks on the French Riviera, Ibiza Island in Spain, running of the bulls in Pamplona [Yes, I ran] and many other good times with two good buddies over the 10 weeks we were there).
St. Pats in Savannah 07 (very cool 4 day weekend)
We have two guys weekends every year, so we tend to go to various cities and party pretty good 2xs a year.
Still on the bucket list (likely to be done in the next 5 years)...
Carnivale in Rio
Oktoberfest in Munich
Fantasy Fest in Key West