I wanted to say Giant Rat, but honestly, Giant Spiders are much more common and probably more iconic at this point.
Went with skeletons because of their versatility in terms of encounter variation, difficulty, and design.
So would you play a game where you fight
only skeletons?
Honestly, yeah, I would. Absolutely. In fact, I'd say that an RPG with practically only variations of skeletons could make for an excellent adventure.
When we're taking CRPG:s, I can't even remember one that was completely centered around necromancy and the threat of the undead. It's sometimes a major aspect, but rarely is it the grand threat. Fuck yes I'd like to see a zombie apocalypse & undeads-R-us necromancy-centered campaign with everything from skeleton phalanxes to mindless crypt-skellies waking up in their tombs, to intelligent liches, both good and evil.
Make it the fall of an entire nation into necromancy, now threatening the peace of the peaceful lands.