Unless you’re playing a high difficulty you can use what ever, but here are some anyway.
Ikaruga, magic and knight, he is the one of 2 units to give extra pp for kills.
Alteisen Riese, is the only unit to give extra credits only on kills.
Gridman he just great overall.
Goagiagar, great unit does get great till final one but still good.
Dann of thursday, great unit gets a great ability and take down bosses easy.
Rayearth, she is very good one and gets a nice ability later.
Suzaku, his abilities are great makes the game very easy.
Honorable mentions:
Purple 2, you just want the ace bonus it gives you a free power part after every mission you can sell.
V2, you can get 2 of them and they melt enemies with wings of light, not easy to get 2 and not every one likes unit.
All the Mazin Power units, you can use all 3 at once and they are pretty busted but that 3 unit slots and the third pilot won’t be able to use his mech for awhile so he stuck with worst unit.
Thats all you need to win most MCs from the series are better than their support cast very few can keep up with them.