Nino. On my first playthrough of the game, I hated her to no end. Second playthrough, it was unchanged.
Present time. NINO IS THE BEST SAGE TO EVER WALK THE EXISTENCE OF FE. I am currently on my 5th *good* playthrough and I'm not bothereing to raise her ONLY because of incorrect supporting and lack of stats on Raven which screwed with my gameplan. Otherwise she would easily end up as a long term unit. I mean, I have NEVER 20/20'd her and she has always, and I mean ALWAYS turned out excellently assuming that I put in the effort. And the fact that I play this on my GBA Gamepak forces me to put in extra effort. But I have twice prometed her on Night of Farewells and 3 times killed Sonia with her. True, Priscilla generally is easier to use because of how early you get her and the fact that you raise her through staves. But the only stat that Priscilla can max at 30 is luck. Nino has Mag, Spd, Skl, and Res that she almost always maxes, and sometimes luck as well. If only she had come in a few chapters earlier. :(
At that, her supports with Rebecca, Canas, and Erk are all fairly easy to get, assuming it isn't a ranked playthrough. Plus, they're all fairly useful supports.
My Sage rankings for FE7-
Nino. Self explanatory, just put in the effort and reap the rewards.
Erk. You get him early on and he's fairly effective as long as his fluctuating growths don't screw you.
Pent. I have to put him last as he is a prepromote. A good one, yes, easily better than everyone's favorite Overused paladin, Seth, but once again, as long as the effort is put forward for Nino and Erk's growths don't jerk you around, he falls short in comparison to the others.
*Edit* To the post above me, the mine trick sounds like an excellent plan.. Maybe I'll try that.