A lot of who you'll use is dependent on what path(s) you choose on your first way through. Also, the order in which you recruit characters will obviously affect who use you at a given time. In my opinion, these are the characters you should look out for:
Magic: Leena, Riddel, Razzly, Harle*. Guile is also pretty decent for magic, but his element grid really kind of sucks intially.
Physical: GrobyC, Glenn**, Leah, Orlha, Karsh, Zoah. Marcy is pretty good, too.
Still, as someone stated earlier, you really can't go too wrong with anybody. This isn't the most difficult game in the world. Basically, you should play with whatever characters you like the most
These characters are really just the ones I find suited to my play style. Hope this helps!
*On your first run through, I recommend not investing EVERYTHING in Harle, for reasons you'll find out. However, in some of my New+ and Continue+ files, she is my primary magic user and kind of kicks butt
**Glenn is especially powerful later on, after you do some sidequesting to teach him something nifty