best choices

Here is my personal opinion. You do not have to follow, but it may help you advance through the game.

I believe that Gilliam needs to be a general. It is one of two you can get in game, and you will receive a great knight anyways. He has the better defense of the two generals.
Colm will become a better rogue than he will an assassin. He will gain stats in that direction.
One of the sword users should become a swordmaster, and the other an assassin. It will give you both, and they can be devastating when used together.
Any of the cavaliers can become paladins or great knights. There are just so many to use and I have not experimented with them all yet.
The dracoknight should be a dracolord. He is the only one you will get.
You will receive three pegasus knights. I prefer to make them dracoknights, but you can change two of them into falcoknights and use a triangle attack. (I never use the triangle attack myself.)
Ross should either become a pirate, or leave him as a journeyman (only if you have beaten the game using Erika once and Ephraim once). The stats gain of a readvanced journeyman make him the best axewielder in the game.
Amelia should become either a knight, or keep her back like ross. (for the same reasons as well.) As a general, she has a better attack than gilliam, but a worse defense.
Your fighter should NOT become a berserker. He will become a greatly reduced fighter.
The hero mercenary should become a hero. He is the only one in-game.
The magic users are a trickier breed. There are plenty of them, so you can go with any direction and still yield every possible magic wielder in your team.
Staff wielders can change to a bishop, or a mage knight/sage. The latter depends on gender.
Larachel should become a valkyrie. She is designed to be one based on her first form. Her stats are indifferent either way, but her attack magic will change with form.
Knoll is not a good mage, but is one of two you can get. This means he is one of the only dark magic users. He will be better served as a summoner because he will not be attacked as often.
Ewan should go for the dark magic classes. He should than become a druid because of the stats Ewan has. (Note that Ewan will lose his ability to use anima magic this way, and will have to be reraised upon becoming a druid.)

Myrrh cannot be changed without cheating. Her dragonstone cannot be mended without cheating. Use her cautiously as she become usless after fifty attacks.
Tythes cannot be changed and cannot fight. She is easily one of the top ten units in the game because of her ability. Defend her.

If you want skills, here they are.
assassin-killer-one hit kill
wyvernknight-pierce-negates defense
general-great shield-prevents all damage
rogue-can do anything a thief can do, but does not need a pick
bishop-always does double damage to monsters

I think ross should be a berserker gilliam a general garcia a hero gerik(not sure how to spell name)hero marisa assasin joshua sword master amelia general ewan mageknight knoll druid other magic any of your choice cormag wyvern lord tana/vanessa wyvern knight colm rouge neimi sniper kyle great knight franz palidan ford I would say don't bother sucks.those are all I can think of so hope you try them.