Tomb of Horrors
My vote is for Tomb of Horrors. Its an orginial Gary Gygax module and just tons of crazy traps and unique situattions. Make the wrong choices and you wind up starting the adventure over buck naked trying to work your way though all the traps. Its an old school high risk high reward dungeon crawl. They redid it for 3.5 and I think 4.0 is coming too.
I also did an update of an old basic D&D module called Horror on the Hill. It took a lot of work but I felt it realy paid off. Module is thin but has a ton of encouters so it's almost a campaign as opposed to a module.
The best 4.0 module I've used I think is the hardback Revenge of the Giants. I'm a big fan of the old "Against the Giants" series and this one, while not as good, caught that old school "epic" feel real well.
If you can't be a good example then you'll just have to serve as a horrible warning.