Are there any sweeter words than "counter target spell" for blue players? This was the original counterspell (as if it wasn't obvious) and it is still glorious. It is near-perfect and it really can't be improved (except by giving it Split Second). For 2 mana, you can stop almost any spell in the game from getting played and there are no restrictions on what it can target. It's one of the most perfect cards in Magic. That being said, there is one teeny-weeny problem with it: its mana cost. Double-blue is something that multicolour deck may have problems getting out. In certain situations (especially in 3+ colour decks), that double-mana cost can be tricky to get out occasionally.
Pros: Hard counter for almost every spells (except those uncounterable spells and those with Split Second)
Cons: Double-blue mana cost may be hard to acquire