There are the "rules" that I created for a very good party

I found that the best way for me fight in the endgame is to have weapons with reach.
Reason: This allows me to fight all the annoying long reach creatures without having to move every turn (forfeiting initiative). If you need a quick spell to heal up your dying party member moving isn't the best option as it will typically let everybody else go before you and unload their dps on your party before you will be able to act.
Also, staying at reach will prevent those long-limbed critters eating your casters alive. If you move to short range they can reach them.

Always max stats. Choose two that you will exclusively race to and then do whatever you like.

With a front line figher I ALWAYS go for strength.
Why? Because then you'll hit like a truck, you'll penetrate every armor (almost always hit), your party load will be higher and you won't use as much stamina compared to speed builds. All these reasons are very important for me. I simply hate when I see that I am tickling the enemy with a non-100-strength character and my 100-strength character mangles an enemy per round. Such a character is hardly justifiable. But then, those occasional stuns and deaths are not bad, but it's just... ...random. In my experience a warrior can typically kill at least one enemy (Rapax Rift comes to mind now) with pure damage while if a class relying on other means doesn't get lucky you might be left with an enemy with 80% health left easily.
Why is load important? I don't like returning to town unless it's absolutely necessary. That means that when I buy consumables (Arrows, stones, sniffing salts(?)...) I buy them in bulk. Add to that that your front liners wear some heavy-duty armor that is not exactly light and you gather things as you go. This can easily become a major nuisance if left unchecked. I was constantly getting into blue load sometimes even more with non-strength characters. Moreover strength increases stamina which is very important. You can last longer in combat, you tire slowly and thus you are longer effective.
Having and not having stamina may often mean in prolonged fights that your party member falls unconscious and enemies will almost certainly kill him. In my first party I have had a very harsh lesson about making speed builds with my engineer. With his fully upgraded rifle he drained ALL his stamina during one round. ...seriously. He WAS freaking deadly, but then he was also very often dead too.
Also strength governs bow, sling and thrown weapons damage too (if I remember corrrectly).

My second choice for melee characters is Dexterity.
There are other interesting options too, especially vitality imo. i prefer Dexterity because it gives me more defense. Vitality let's you soak more damage. That means moar mana drain on healer. Iron skin IS nice, but that's all it does. More damage soak, better stamina and I thing slightly better load. The thing is that I don't need that extra hp in endgame. Dexterity at 100 gives you nice damage mitigation on itself PLUS more attack (so more damage) and I think it improves hit mechanics (To Hit?).
Going for vitality with warrior will create interesting situations. Hell, it even saved me a lot of times when I had that build. You may actually destroy all the enemies 10 rounds after the rest of the party died but the ressurection powders are so expensive... ;) And I sell most of them anyway.
That being said vitality IS a good stat. I just prefer Dexterity.

Vitality:
Good to raise after you have reached your two primary stats goal. Where at melee characters you struggle to choose the the "right" stat, Vitality is a huge deal for casters. Seriously. Go for your main stat and vitality. Game will be a breeze. There is a serious difference between a 100hp mage and 200. You might even cast a few spells in the endgame before you die. I ALWAYS go for vitality + piety with my healer. He has to stay on his feet and those two stats (along with damage reductions at 100) do exactly that.

Piety:
I take it as a main sat for priest ofc and I try to get some with other casters after I have reaached my first two 100-points-stats. Iron will is fabulous.

Intelligence:
Now intelligence is to offensive casters to what is strength to fighters. And I can't stress this enough: If you don't have Power casting in the endgame you are ROYALLY SCREWED. Your mage/psionic/whoever will be reduced to support role and try to get some cc on enemy because his damage spells burn enemy Rapaxe's eyelashes and itch him to annoyance at best. I have made this mistake once and I will never do it again. Almost every monster later on have 100+ resistance to spells (With the "weak" ones at 80). Must be.
Intelligence is a very nice choice for 2nd set of maxed stats for non-offensive casters.

Speed:
While nice stat in my experience it's not a good choice. You have more important stats to choose from and this is a 2nd-round choice. Snake speed is rubbish imo (expecially when you need to move in the round). Melee characters with too much speed tire so quickly (you probably don't have enough stamina since you have chosen speed over a stamina-abundant stat), casting that fireball before the enemy is not that vital in the long run. There are moments when it comes handy, even might win you the day but would I choose it over Intelligence, Strength, Dexterity or Vitaliy? No. 2nd choice? Perhaps.
The only moment when it comes handy is a quick heal before enemies can act but then my priest doesn't exactly prioritize speed.

Senses:
Wasted stat. Except for ranger. You CAN choose one character for it for searching purposes but I wouldn't build a strategy around it. Now if you wanted a ranged party, that would be an entirely different story...


Races:
I don't have any strong preferences except I take dwarf as my priest and I choose something with high starting strength for my warrior (Lizard). This is because once I have my maxed warrior the rest of the party can slack. Really. They are THAT good.


Classes:

I always have a priest.
I always have another caster for buffs (usually a mage).
I almost always have a warrior when testing new stuff as whatever I screw up I know that all I need is that warrior and enough time. Warrior is the most overpowered beast in the game in my experience. It's an unstoppable killing machine. Especially in the beginning... Monk: Bam 6 hit, swoosh 3 hit, Valkyrie: -thrust- 13!! hit, Warrior: 40-freaking-kaboom-heads-and-limbs-flying-around-next-please!!! This doesn't stop even in late game. Hits over 100 are a norm for him.
Keep in mind that your hybrid won't do much damage with spells in endgame if he doesn't have Power casting. In this having a samurai with intelligence 100 (And Power casting) makes a lot of sense if you plan to hurl fireballs at Rapax.
I try to have 3 characters that can stand in melee.
Rest is up to what i feel.

Conclusion:
1st spot: Warrior. Go for Strength and Dexterity.
2nd spot: Priest. Piety, Vitality.
3rd spot: Mage. Intelligence, Vitality.
4th spot: Melee with reach and possibly utility. Valkyrie with halberd is a very nice choice. Offheals, can survive death, has bonus on halberd and wears heavy armor. You can go Strength Dexterity with her. She's not an offensive caster by nature (although you can get nice priest spells later on too).
5th spot: You might want 3rd melee with reach that fills in the magic school you lack. Monk can be a very nice choice. But don't worry to experiment.
6th spot: Ranged or caster. Again, based on what you lack. I tend to use ninja here as they can open locks (although i suspect it's redundant with casters and strong warrior), cast spells and have critical on thrown weapons. Just make sure not to waste too many points as you can get jack-of-all-trades who can't do anything properly.


I ask the readers to take into consideration that this post was written based on my experiences and represents my opinions on the game. If you have a different one and disagree with me, do so in a polite and constructive manner, please. Explaining your opinion is a great first step towards a dialogue. :)