I like Mallow for his cheap multi-enemy strikes, like the Lightning Storm. It's actually pretty decent later in the game, especially if you hit the A button right after the last bolt hits the ground. Overall, I found Geno pretty unreliable. My party generally consisted of Mario, Peach, and Bowser.
I had the player's guide back when SMRPG first hit stores, and I gotta tell you--it's one of the most error-ridden player's guides out there. If has a section dedicated to where all the hidden treasure chests are, but a few chests are accidentally repeated. So out of, say, 50 chests, you only find out where 48 or 49 are. :-( Also, there was an enemy list with HP, SP, weakness tables, etc., and more than half the time, SOMETHING was wrong. I remember distinctly that according to the guide, Mack's weakness is fire. It turns out that's totally wrong--he's strong against fire (he uses fire attacks) but weak against Jump.