I recommend having one from each type in your party - a mage-type (Red), a warrior-type (blue) and a rogue-type (green). I would also be sure to have at least one healer and one spellcaster.
Healers (in order of effectiveness):
Druid
Paladin
Warlock
Harlequin
Ranger
Spellcasters (in order of effectiveness):
Wizard
Warlock
Druid
Mesmer
Ranger
Black Knight
Harlequin
As long as you have healing and spellcasting covered, I wouldn't worry too much about your party make-up, though. All of the base classes are effective in their own way, plus you get to make 4 different parties, which encourages experimentation. Furthermore, you can discover secret classes AND upgrade to elite classes (using elfstones) in chapter 5.
This is my tiered list of all the classes (including secret, elite and hero classes).THIS IS JUST MY OPINION and I have heard reports of other classes being used much more effectively that I did. After you play the game, let me know what you used and what you found to be effective. I'm interested to know!
GOD TIER
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Daredevil
Dual wield + Strike (x5 or double damage) is devastating. With two Ultrawhips (x2 attacks) with a x5 Strike, you can get 20 attacks per round for nearly 8000 damage. And this was with a Level 23 Daredevil in the final dungeon. Also, superfast AND has access to all the Ranger, Ninja, and Bandit skills and all the songs.
Hunter (Holdana)
Pummel at higher levels can do x4 or x5 attacks per round. Combine that with kickass hero-only equipment AND competant spells (like Thordain and Poison and healing and revive spells) AND that Hunters naturally have a 50% resistance to elemental attacks == WIN.
Templar
Has Pummel (see above) AND Chainmagic (x2 spellcast/round) AND an awesome spelllist AND crazy HP AND able to equip everything that a Black Knight can. Basically like a Hunter.
TOP TIER
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Magus (Zefiryn)
Has Magicombo (x2 spellcast/round) and Thordain. Plus can turn into a Weremage (x2 INT). Remember Terra from FFVI? This is basically the same thing. Instant Demon Death.
Channeler
Casts powerful magic without needing MP.
Champion
Has Pummel and can equip awesome stuff and lots of HP.
Sage
Has Chainmagic and a good selection of powerful spells.
MID TIER
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Shapeshifter
Has werewolf form (x2 ATK/AGL + regen), powerful weapons, good HP and insane STR. Equip the Blade of Arborlon for 3000+ damage mid-Chapter 5.
Ninja (Akira)
Throwing weapons are good early and mid game weapons, but become increasingly useless as the later game develops. Still has hero-onyl equipment and spells, though.
Monk (Raj Ahtan)
Same hero stuff, plus dual-wield and has some good healing skills, but lacks any real special talents.
Warlock
Alchemy FTW, plus revives are usefull.
Wizard
Lots of good spells early, but is outdone by the Sage, Channeler, and Templar.
Ranger
Lots of good spells, too. More useful if the Training Guides are applied.
Druid
Healing spells are a livesaver, and the earth spells are good, but not much of a damage dealer.
LOW TIER
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Mesmer
Demi FTW. Plus, Bikill (x2 ATK) and Focus (x2 INT). Makes other party members awesome, but is not so awesome himeself, especially late-game.
Black Knight
Pretty strong, lots of HP, good equipment, but lacks a powerful punch. Has some low level spells, though.
Ronin
Dual-wield can be effective, but spells are lacking, and Toss is underpowered (but too hard to fix! I'd have to change every single battle event).
Bandit
Useful early on, and stealing is fun, but doesn't do much damage, nor adds much to battle skills.
Harlequin
Tarot cards can be very powerful early and mid game, but loose their luster late game. Heroic Ballad is awesome, though.
Paladin
No real damage dealing ability, and healing skills pale against other classes.
Engineer
Tools are good early on, but loose effectiveness later.
JOKER TIER
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Joker
This is basically a goof-off. They can become Daredevils WITHOUT needing an elfstone after they reach level 20 or so.
REMEMBER, THIS IS JUST MY OPINION, and like all tiers, completely subjective.