for me i liked to use:

Zidzane (duh)
Steiner (he is very powerful and i dont know if there is anything bad about him)
Vivi (black magic is awesome, doomsday kicks ass ...and vivi is cool)


now id recommend Amarant for the begining-ish
but later itd be nice to use Garnet and make her learn all the Summons
cas she is very powerful, especially Ark (in my opinion better than Eiko)

Why only Amarant in the beginning?
Because he is basically like Steiner except less powerful
Garnet has special features that Amarant does not have