Race Based Character Suggestions
 

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:

Suggestions: (Also including low warrior and warlock for max HP where not included):  

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:

 
 
Giant:
They can only join Warrior, Explorer and Barbarian - and are usually found as characters in all these guilds.  Barbarian gives a low magical entry cost and backstabbing ability, Explorer gives movement spell ability, and Warrior allows them to flaunt their natural strength. Usual order of importance is Warrior, Explorer and Barbarian.  Barbarian helps other characters with thieving, and also prevents the character from being robbed blind by stealing monsters.  Give them high constitution to begin with to maximise their HP (as they are usually frontline fighters), then start building up their strength.
 

 
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:

Suggestions:  
Dwarf:
Dwarves can only be neutral, allowing them to join up with any character or party.  They are quite versatile, and the combination of their good statistics as well as being able to join four good guilds results in dwarves being quite popular characters.

Guild not to join:

Suggestions:  
Ogre:
Due to the severe guild limitations of ogres, they are only useful if played as neutral or evil, allowing them access to the barbarian guild, otherwise their spell points go to waste through the high cost of magical entry in the artisan's guild.  Ogres are generally neutral, as this means they can join with good or evil characters.  Make sure that their CON is as high as possible, as having the highest con, giving them high HP is one of the few things ogres have going for them.  So make them neutral, high con, and go kill something... :)
Ogres are great as a first character to collect tomes and potions for your other characters, as they don't have a lot of potential to travel to the lowest levels of the dungeon.
 

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:

 

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:

 

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.