Elves get picked on a lot for all their subraces, but the fact is the PHB elf just sucks for just about any build. Might do an arcane trickster with it, but not much else.
Now, the MM elves are more viable. The gray elf gets a bonus to Int instead of Dex, giving them an actual edge in their favored class of wizard.
The wood elf's ability score bonuses are wonky, but they do make for a good archery-based ranger.
Dwarves, OTOH, don't seem to need another subrace. I mean, who chooses to lose Dex instead of Cha?
Gnomes? Those guys are just a mess. Too many stupid little racial traits to keep up with. Desperately need streamlining.
I allow not only subraces, I allow monstrous races. I've had a rakshasa, a minotaur, a thri-kreen, a frost giant, a desmodu, couple of catmen....some worked out better than others.