It seems to change as time passes. With the stats available on gear now, people don't seem to be as picky about having the "best" race for their class anymore though.
From my experience from the FV and MS servers:
Warriors-Wood Elves, gnomes and halflings seem rare.
Paladins-Gnome and Erudite are rare(High Elf used to be years ago).
SK-think gnomes would be the most rare
Ranger-nothing really rare, but you see more WE than the other races
Rogue-Gnome again(I don't ever remember seeing one but Caster's realm says its an option)
Bezerker-think I've seen the least Vah Shir, but I have 1 myself
Shaman-I would say Iksar is the least played Shammy race
Bard-I've seen fewer Human bards than the other choices
Monk-N/A since there are so few choices
Cleric-Gnome, halfling, Erudite all seem pretty rare choices
Druid-Half-Elf is probably the least picked race, followed by Human which is also pretty rare for a druid
Beastlord-Troll, ogre, iksar all fairly rare...I'd don't remember but 1 troll beastlord in all my years of playing.
Now you would think the caster classes would all be pretty similar, but...
Enchanter-I don't remember any Human or Gnome chanters
Mage-I only remember playing with 1 Dark Elf mage
Necro-Probably Froglok, since it was a late addition to the Frog class line-up
Wizard-Froglok
My votes for the most rare race/class combos:DE Mage, Troll Bst, Gnome Rogue, Gnome or Halfling Warrior, Human Bard, Gnome/Halfling/Erudite Cleric