Microdot

I first came across Microdot as a freebee on a cassette attached to a magazine, which also included the excellent Spots, a Yahtzee clone. Microdot caused me to break at least one joystick, stay up well beyond midnight trying to finish the game, bleary eyed and frustrated; for try as I might I never managed to get more than about 30 particles.
For a freebee, this game is amazingly huge, with just enough easy/difficult ratio, so that despite the frustration, you have to keep trying again and again and again.

The concept is so simple - you are a microdot with the awesome mission of collecting 99 particles to diffuse the radioactive chip - or something - I never got as far as that. Along the way, you have to avoid coloured sprites, background radiation and electrical discharges as these would cause your poor little microdot to overload. To help you along the way, you have 'superjumps'; and bonuses in the form of flashing triangles that will boost your score, give you extra lives or superjumps or even provide temporary background or sprite protection. No chance of finishing the game too quickly, there are 225 screens if memory serves me well.
OK, so the graphics are dated now, but just remember this worked on a 64K machine, whereas nowadays a mediocre game starts at around 2 meg, the best ones 10 meg or more, just making you wonder how we ever got satisfaction out of this stuff - but we did.
If that wasn't enough, the game contains a level editor, allowing you to build and try out your own fiendish screens, though I never had the patience or the imagination to build more than 10.
Perhaps the most frustrating part of the game is the fact that that screens 'reset' every time you enter one; for instance, you may have to make sharp evasive action back into a previous screen as a sprite comes hurtling towards you, when you go back again the sprite is back at the starting position again, or if you're trying to jump over something that goes into another, upper screen and as you land you're in danger of landing on the sprite's starting position - annoying, but you haven't paid out good money for annoyances, no money at all in fact, for what I guarantee is hours and hours of fun.