Been digging through my collection of old ZX Speccy games looking for things to speedrun or just stuff that aged well.
Elven Warrior - looks like a cutesy platformer, but you being unable to jump sideways turns in into more of a puzzle where the main challenge is to find an optimal route through each 'dungeon', spending as little ammo and spells as possible.
La Espada Sagrada - don't speak a word of Spanish, but this one is impressive. You've got Dizzy-like inventory puzzles, fiendish platforming, and an entirely different game as the last level. With a bit of a stress on the 'fiendish' part.
Hydrofool - one of the better isometric collect-a-thons. The wonderful Rob Hubbard's tune is both somewhat out of place and weirdly appropriate.
Moontorc - also a platformer with limited resources. Also with focus on melee, shops, coloured keys, and weird jumping physics. Could be fun to speedrun.
Myth - a few neat tricks and skips in this one, speedrun-wise, but having to collect 10 skulls at the start, along with the random drops, kills the speedrunning potential. Pretty fun levels casually, though - plenty of gimmicks to figure out.
Pixy Microdot 2 - I just marvel at the mere fact how much character can be in a game where you're controlling just one pixel. Some tight corners in this one, with magnets and inertia to fight.
Rex - sat down to play this one properly at last, and boy have I never expected it to be this hard. (But damn it looks gorgeous even now.) You're trying to preserve what little shields you've got left, whilst making tight jumps and trying to avoid crossing too many lines of sight. And then you realise there's side B.
Rockman - part Lode Runner, part... Boulder Dash? Sokoban? Quite addictive, despite being a rather early title.
Wacky Races - the racing segments are a pleasure to route and execute, if a bit heavy on memorisation. Some of the nicest platforming on the Speccy. The game as a whole is a blast casually, might also make for a good speedrun (although I fear that with proper execution it could be a bit auto-scrolley).