As a blue mage myself that plays almost exlcusivly in group games, so i have a thing or two to say. First, I can only afford to counter things that represent an immediant 'i win' for the caster, and things i can't possibly deal with later. If somebody were to play a lanowar elf, i wouldn't counter it. They could probably hit me with it for a good 5 turns or so before i find something to stop it with. In a group game i am drawing 1 card for every 3 or more the rest of the table is drawing. That's horrible card disadvantage, luckly most of those cards get used up on each other, but there are still 3x the number of real threats that i *need* to counter as in a duel.
Anyway, My rating(from worst to best):
Red: burn. yeah. mass destruction is it's one redeeming feature
Green: I find most of the cards boring, but there are a few that shine(enchantress, birds, verdant force, plow under, etc.)
White: Protection is neat, and angels, but saving creatures is still worse then killing them, imho.
Black: Discard, creature kill, life draining spells(way better then either dd or life gain).
Blue: Easily the most complicated color requiring the most thought, imho. Ways to deal with *everything*. card drawing, bounce, counters.