I'm making a 5e world and I don't think I want multiple subraces for each race. So, given that, which subraces do you find the best at suiting the archetypes of the race while also having solid and/or interesting mechanics?

Dwarf: I'm partial to Hill Dwarf as I find it far more versatile than Mountain Dwarf.
Elf: I can't decide whether High Elf or Wood Elf is more fitting.
Halfling: I don't really have a preference, but I've only ever seen Lightfoot in play.
Gnome: While neither is among my favorites, on the mechanical side, I like Forest Gnome way more than Rock Gnome.

Generally speaking I like your choices. High Elf has a more classic dnd feel. Wood elf is mechanically very good, but something feels off to me about it. If you don't have a half elf in the game, I'd consider switching it out as the elf race's mechanics.

You might want to poll your players. If you only want one subrace each, then the "best" pick is the player's PC choice. Alternately, you might want to check out 3PP resources (and wikis) to see if there are any new subraces which might be fun to build your world around.

Why not let setting scope affect demographics? i.e. Underdark: Grey Dwarf, Dark Elf, Deep Gnome, Earth Genasi...

Or define the world's conceits and aesthetics first and let it all all fall into place? Paladins & Princesses? Classic versions of each, as per 5e Basic already.

I think my issue with the 5e gnomes is that, while 5e tells us that rock gnomes are the "standard" gnome, the abilities of the forest gnome--such as minor illusion and speaking with animals--fit much more of the early D&D versions that I liked (i.e, Greyhawk but not Krynn) of gnomes than the ability to tinker and make toys (which I also think is a very poorly written ability in terms of mechanics).

Since I'm not at all interested in a "tinker" type of gnome or even gnomish "crazy professor" alchemists, I don't lose much by eliminating rock gnomes and making forest gnome the default. I will also make the cosmetic change of having the Gnomish language use the Elvish script (rather than Dwarvish script) to highlight closer ties between gnomes and elves rather than gnomes and dwarves. It doesn't hurt that Elvish script is also used in Sylvan, which I would imagine would be a fairly common language choice for these forest gnomes.

Hill Dwarf seems as silly as Rock Gnomes.

Keep it simple.

Quote from: Theory of Games;1098253Hill Dwarf seems as silly as Rock Gnomes.

Keep it simple.

Why is Hill Dwarf silly? They are the ones that are naturally more intuitive and robust while Mountain Dwarf is all about muscles and wearing armor. I generally prefer Hill Dwarf mechanics (and less isolationist flavor) over Mountain Dwarf for almost all character types. Even missing out on the higher Strength of the Mountain Dwarf isn't really that big a deal for many character types and those that need armor proficiency can typically get what they need from their class.