Pass on Guitar Hero World Tour, it has a terrible vocals engine. You literally have to hit every tick of every note to get a 100%.
Every game past that is fine (with the exception of Guitar Hero Van Halen, which shares that engine). Rock Band 1 and the track packs supposedly have a wonky engine but I've never encountered a problem with it. 2 is fine. Beatles and every game with harmonies are great for parties. Kepp in mind, however, that Rock Band 3 lacks competitive multiplayer, and, as such, you should take that into account when making your purchase.
I'd also reccomend Karaoke Revolution, apparanty it's not that bad.