I've tried all of them. Green is the best. Blue will do when you can't find green. Next best are orange and yellow varieties. There's a new one - Zero Sugar which tastes pretty nasty. And there's the "Dry" kind which look like this:
The green nitrous one tastes like orange peel. It's weird.
In the US, green monster tastes slightly different, and they sell the EU kind as Monster Import.
It has an awesome resealable lid, good for when you don't want to drink it all at once. When I'm in the US I sometimes drink the regular green kind and sometimes the import.
The only ones I won't drink are the coffee kinds, as they have milk in them.