Blue. I ALWAYS try to work an enemy-skill user into my party in every game. In my last play of V, my final party included Bartz as a Blue Mage backed up with Time abilities. Awesome. My last play of VI involved my first ever use of a beefed-up Strago in my final party (I was going for a 3/4 magic-using party). In VII, I usually turn Tifa into a physical powerhouse equipped with a nearly maxed-out Enemy Skill materia. I keep green materia off her too to make sure her STR stays as high as possible. She's always my most efficient killer. When I do play VIII, I use Quistis because a) she's hot, and b) I like the very simple way of acquiring Blue magic. For that same reason, I don't really use Quina in IX. Hate having to reduce the enemy down to minimal health and then eat. Annoying.

But yeah, blue magic is great because it usually includes Black, White, Green, Time, etc somewhere in its library. None of my parties are complete without a blue guy.