I have everything from Shimano 600, Alivio, RX, 105, Ultegra, and Dura Ace. From 6 speed to 11. Maybe some I don't remember. I've never met a Shimano group I didn't like. They all work flawlessly when adjusted properly. Buy what fits your budget and enjoy.
edit: PS The cheapest, the Alivio, on a mountain bike has been through several thousand miles of commuting, through mud and deep water. Everything is original and still works perfectly.