My fave's Glenn, for several reasons.

1.) When you first run into him, he just has the aura of mystery about him that only Magus seems to contend with.

2.) He's weak magically, but if he has some of the best TECHS in the game (that's techs, the non-star moves). I know some people will argue that Ayla has the best techs, but her most powerful techs are very expensive on mana. A Gold Stud would remedy it, but Ayla with a Gold Stud is almost an insult compared to Ayla with Prism Specs.
Glenn, Meanwhile, possesses some decent power for relatively low mana cost. Leap Slash is overkill by itself, but with Dual Techs such as Spire and Red Pin it deals a fair amount of damage for a reasonable amount of mana. And Double Cure is downright awesome. It heals the entire team more or less entirely for almost no mana.

Hint for Newbs going after Lavos for the first time: If you stick a Gold Stud on Nadia (Marle) and a Silver Stud on Glenn (Frog), then just keep casting Double Cure over and over again while Crono wails on Lavos with Rainbow, you'd have to be pretty weak to lose. On subsequent runs with New Game+ you can get additional Gold Studs thus making your Double Cures never ending.

3.) Aside from Ayla and Magus he's actually pretty swift, and he has the advantage of physical strength over Magus and cheap (mana-wise) techs over Ayla.

4.) His story line rocks, his speech pattern rocks (I irritate my friends by speaking the same way Glenn does), and his main weapon is the Masamune. You just can't argue with a weapon that is powerful against so many enemies. Magus, Zeal, The Mammon Machine, Lavos. All of the "major" bosses, the Masamune beats their fat nazi butts into the ground.

5.) Theme Music. Aside from maybe Robo, he has by far the coolest Theme Song.

6.) He gets married to Leene! That makes him Nadia's Great-times-ten-Grandfather! EW!

I'm not saying that what I'm saying is right. I know that many people will say Magus is better for sheer power and speed, Ayla is better for the same reason (plus charm), but this is just my OPINION. I'm not trying to jam this down anyones throat. If you feel a desire to disagree with me, go right ahead