Note that these suggestions are just based on how valuable they are to a character succeeding.  While I may suggest that a character does not join a particular guild or play a particular alignment, it does not in any way mean that it would not be fun to do so - I think the game is fun pretty much however it is played. :)  Also note that in all character suggestions, characters are assumed to be members of the artisan's guild.
 

Human:
Multi-class, multi-class, multi-class! Often found as good or evil, allowing them to join the ninja and sorceror guilds - which is probably the most powerful combination of two guilds in the game.  All  human characters should be given maximum HP by using warrior, artisan, warlock maximum HP route, and focus more on fighting, or evenly on fighting and spellcasting.

Guilds not to join:

• Explorer - gives heavy penalty due to fighting ability, and spells could be obtained with less penalty from Warlock.
• Paladin - fairly poor fighting ability, and healing ability is outweighed by either the Magi's guild(with charm) or Cleric's guild.
• Villain - Heavy penalty, and a ninja/sorceror combination is far more powerful.
• Barbarian - Stupid if neutral, as thief is far better, and a waste of time if evil, as it will give a moderate penalty for only decreasing the cost of magical entry.

• Good/Evil  Ninja, Sorceror, Magi.  Well rounded abilities.
• Good/Evil Ninja, Warrior, Warlock.  Does lots of damage in combat due to multiple swings and fighting ability. Warlock's spells are also a great complement, but warlock will suffer heavy penalty.
• Good/Evil Ninja, Warrior, Sorceror.  Similiar to above combination, but the damage and resistance spells could be considered more favourable than the movement and other spells of the warlock.
• Neutral Warrior, Thief, Cleric, Warlock.  While the best neutral characters are probably dwarves or saris, this is still quite a good character.

Elf:
To put it in slang - Spellchucker. Elves undoubtable make the best spellcasters in the game - so get that INT and WIS up and start blasting those monsters to bits!  They are often good so they can join the Paladin's guild to give them some fighting ability in case their spell points happen to run out.  Elves are also usually good or evil, allowing them to join the sorceror's guild, but neutral elves are still quite powerful.

Suggestions:

• Good  Sorceror, Magi, Warlock, Paladin.  Can fall behind on exp., but is very valuable in moving around the dungeon and casting offensive spells.
• Good  Sorceror, Paladin, Warlock/Magi.  More fighting ability means this character is more offensive and will gain more exp., but the movement spells with suffer.
• Evil  Sorceror, Magi, Explorer.  Explorer will still give reasonable fighting ability, and only a small amount of offensive capability will be lost with the removal of Warlock.
• Neutral  Cleric, Thief, Warlock.  Keeping cleric with the highest level with reduce its penalty, as well as increasing the potency of the healing and damage spells.  Valuable for opening chests and moving around also. Backstabbing ability of thief will result in some usefulness in combat.

 
Gnome:
The supreme masters of charming enemies, gnomes are virtually always found in the Magi's guild, and their high charisma means many monsters will simply join them without having to be charmed.  Having a higher CON and STR than elves, they tend to suffer less from lagging behind in the XP percentage.

Guild not to join:

• Barbarian - thief is better if neutral, and villain is more rounded for an evil gnome.

• Good Magi, Paladin, Sorceror.  Good at charming with decent fighting ability.  Offensive sorceror spells are also valuable.
• Neutral Cleric, Magi, Thief.  Similiar to the Elven Cleric, Thief, Warlock, but with companions could be considerably more powerful.
• Evil Magi, Sorceror, Villain.  Villain will give a penalty to Magi and Sorceror, so they should be considerably higher in level.
• Evil Magi, Villain, Explorer.  Explorer could be removed entirely, or just gained up to obtain minimum cost movement spells.

Guild not to join:

• Barbarian - A combination of thief and warrior levels is far better, both for fighting ability and for chest opening/trap disarming spells.

• Warrior, Thief, Cleric, Warlock.  Joining these guilds, this character might level slowly, but has virtually all the skills required to survive the dungeon solo.
• Warrior, Thief, Cleric, Explorer.  Will lack some spells(morkal death & high resistances) but will have a far better perception.
• Warrior, Cleric, Thief.  Eliminating warlock/explorer means loss of movement spells, but the increased leveling in cleric (and hence healing and damage spell capability) could offset this.
• Warrior, Thief, Warlock.  The loss of healing spells will hurt this character's solo ability, but they will have strong fighting ability, backed up with movements spells to quickly return to town when injured.

Yeti:
Quite dextrous, but ugly, Yeti are valuable because of their great underwater breathing capability.  They can join the magi's guild, but given their low charisma they may not be able to cast some of the more powerful charm and bind spells(Unless aided by charisma increasing items).  They are quite effective warriors though, having a good STR and DEX, backed up with sorceror capability.

Suggestions:

• Good/Evil  Warrior, Sorceror, Explorer.  Powerful fighter will offensive capability and movement spells.
• Evil  Sorceror, Villain, Explorer.  Same as above, but villain may provide a more rounded fighting ability.
• Or drop Explorer from either of the above combinations so leveling can be faster in the other two guilds at the expense of movement spells.

Saris:
They can only be neutral and only join six guilds, and are often found in the same guilds as dwarves(Except they can't be warriors).  However, Saris have the huge advantage over dwarves of having a brilliantly high dexterity(Max natural 28 DEX).  Though they have less STR and CON, their tremendous DEX is very popular since they usually get first strike in a battle.

Suggestions:

• Explorer, Thief, Cleric, Warlock.  Go the whole hog and use all four guilds.  Slow to level, but pretty good.
• Explorer, Thief, Cleric.  Drop warlock and lose a few spells in order to gain levels faster.
• Thief, Cleric, Warlock.  This character will lack fighting ability comparatively to those in the explorer's guild, but should level a little faster than the second option, and will have morkal death spells.

Troll:
Trolls can have three alignments, but virtually only found as evil, due to their allowed guilds.  As mentioned in the race overview trolls are only really useful as Evil Ninja/Villain/Barbarians.  Neutral thief is a waste, as is a Good Ninja.  The ninja's great fighting ability combined with the better thieving and backstabbing ability of the barbarian, and villain for some spellcasting capability make these trolls a powerful character.