Trilogy? Lord of the Rings, hands down. If you can get all three extended editions, you've got 12 hours of watching material.
Other options would be like others have mentioned; Jurassic Park and the Bourne Trilogy. Also, Pirates of the Caribbean. People have most likely already seen all those films, but they're pretty cool as well.
Oh, and as a bit of an underdog, there's the three Underworld films. Probably not the 'best' trilogy ever, but if all company is into that kind of semi-supernatural stuff I'd pick it above the other options I've just named.