Mine has to be the Slyth from Underdark.
Dopey-looking humanoid-ish slimes that get along well with most people, act as the underdark janitors, and have the ability to get into places others can't get to easily with a +2 to dex and Wis and a -2 to strength? That's just begging to be made into a druid, bard, or rogue. Additionally, it's always nice to have some of those perks like:
- Alternate Form: a moving ooze(most ooze benefits and penalties) or slime-man(default form)
- Water Breathing: Campaign dependent, yes, but there are lots of lakes in the underdark so it could come in handy.
- Immunity to polymorph and poison
- Sonic Resistance 5: Not much, admittedly, but it's not a very common resistance and it tends to add up.
- +4 Racial Bonuses to Disguise, Escape Artist, and Survival: useful, if not all that common.
- Weapon Familiarity(Flutter Blades): Admittedly not too good( 1d4 for medium, 19-20 x3 crit, slashing or piercing and switch between with a move action), but it's free so can you really complain? Even better if the DM lets you switch it out for something else (like gnome quickrazor or spiked chain).
My biggest gripe with the race, however is the +2 LA, but that can be reduced with LA rules from UA. For those bonuses it can be a bit hard to swallow at first glance, but it has no HD, a good amount of ooze traits, the ability to switch between ooze and humanoid form as a standard action, and the above benefits for 2 LA that can be reduced. I'll take that.
Plus if Inevitability ever does Underdark, they might get a lower LA, so that's another selling point.