Mario Party 8 is the best, my friends and I can't go a week with out playing a game. I got it the day of release and we're still going strong with it.
WarioWare is pretty good, but you have to unlock multiplayer mode by playing through the singleplayer game (The last level can be tough too!). Still an awesome game though.
i think wii sports has to be the best party game because everyone either likes tennis, bowling, golf or boxing..but for a 5 year old birthday party i would suggest mario party 8. older party wii sports for sure