For myself...
Lupins: Fun fact is, I came to D&D very late in the game's lifespan. Just the wrong combination of hometown, country, birth-year and being suckered into buying Warhammer Fantasy & 40K codexes instead of D&D books. So, most of what I know was born out of searching for D&D stuff online. Thus, my first exposure to Lupins was in their 3.5 unofficial reboot in Dragon Magazine #325. At the time, I read them and just went "huh, neat", and would have passed them over - I mean, giant wolf-riding Great Plains Amerindian-themed wolf-people is not really that unique. That's, like, one of the more cliche cultures you'd get in most anthro fantasy settings. Except one small sidebar (criminally missing in Dragon Annual) caught my eye. Pictures, evidently taken from old school D&D sourcebooks, introduced me to the kingdom of Renardie, and that caught my attention. Faux-French Swashbuckler-themed wolf-people? Now that is something unique! Heck, the division between Civilized, Tribal and Nomadic is practically tailormade for 5e's subrace mechanics, although as one who's tried to convert them, I must confess that I struggled with meaningful stats, given their rather lackluster mechanics in previous editions.
Aranea: My interest in the Lupins led me to investigate the Red Steel setting. Although incredibly obscure - WoTC hasn't even released it on DriveThruRPG, and they just put out Hollow World in print! - I managed to track down a copy, and they introduced me to the Aranea. For many reasons, I find this race fascinating. Not only is the basic concept - shapeshifting, sorcerously-adept spiders - unique, but most bug-races tend to be evil or at least a very hostile form of neutral. Aranea, despite this, are not evil in the slightest, and are presented as just another race, as normal and as accepted as the dwarves, elves and gnomes. This is a rare thing indeed, and truly makes me want to see them return.
Diaboli: Despite the fact these first three races all come from Mystara, I honestly don't know all that much about the setting, and I consider myself more of a fan of the Poins of Light setting. Still, the Diaboli are an incredibly fantastic race and I'm surprised they've been allowed to go so obscure. Their entire schtick is that they're a race of extraplanar migrants fleeing after their home plane (the Realm of Dreams) was invaded by horrible monsters from yet another plane (the Realm of Nightmares), and they're struggling to survive on a new world because, despite a typical racial alignment of Chaotic Good, they look so much like stereotypical devils. I just think that these would be an excellent race to add in a new extraplanar-focused sourcebook.
Deva: In general, I must preferred 4th edition's fluff to 3rd edition's (and thus 5th edition's). Whilst the 5e Aasimar is more unique than it was, with the whole "guardian angel" bit, the Deva remains more intriguing to me. A race of non-evil Fallen Angels, beings from the Astral Sea who chose to give up their great power as immortal servitors of the gods because they felt the lives of mortals were worthy of embracing. That's a powerful and unique racial concept, in my eyes, and I'm really sad that it seems likely Devas can no longer exist with the new metaphysical fluff used in 5th edition.
Shardminds: These were effectively the first race truly unique to 4th edition, and unlike the poor Deva, their connections to the Far Realm means that they can successfully be brought over into 5e. These work wonderfully for both psionics-focused settings and for extraplanar settings, and have a unique niche as literally guardians against the aberrant hordes.
Thri-Kreen: Who can forget the most iconic of races from Dark Sun? As much as part of their appeal was due to munchkins exploiting their ability to quadwield in an edition and setting when melee masters were king (so long as high-level wizards were off the table, anyway), it must be emphasized that the core of it was how unique they are in both comparison to the standard races and in that they're a bug-people race who are neither inherently evil nor bound up in oppressively collectivist to the point of hive-minded culture.
Gnolls: I love gnolls, I just, can't explain it better than that. They're usually dismissed as just furrier, tougher orcs, but I think there's a lot of potential in them. 4th edition gave them wonderful fluff in "Playing Gnolls" in Dragon #367, emphasizing their nature as a race torn between the hyena's soul and the demonic influence in their veins, a race inclined towards savagery but capable of embracing the light. They're a race with so much potential... which 5e took out into the back alley and shot. I can't emphasize enough how much the Volo's Guide fluff for gnolls angers me. I want my gnolls back. They've been playable in every edition - even Basic made them playable in the sourcebook "The Orcs of Thar". I see no reason why 5e should shoot that particular sacred cow.
Shadar-Kai: This is another race that I feel 4e really made interesting. Fitting into setting that has the Shadowfell or a similar "land of darkness/death" plane as part of its cosmology, the shadar-kai are an excellent "Dark is Not Evil" race, with an interesting culture and a built-in reason to be adventurers. Lot of potential here, and I'd love to see WoTC bring them back.
So... that's my little kneejerk list. What's yours?
So, I ask you all in sincere faith:
