I loved my Spectrum and played 'Jetpac' to death but it wasn't long before one game came to dominate above all others, the absurdly addictive 'Manic Miner'.
Created by Matthew Smith, Manic Miner was a platform game where you took Miner Willy through a series of 20 levels collecting flashing objects and avoiding a variety of weird creatures such as ducks, jellyfish, toilets, leggy pacmen, kangaroos and seals balancing balls on their noses all on pain of running out of air or bumping into one of the marauding critters losing all your lives and getting squished by a Monty Pythonesque shoe.
It went on to spawn a number of sequels but none of them managed to capture the sheer, addictive fun of the first (although 'Jetset Willy' came close) and some 30 odd years on it remains a ridiculously playable game.
You'll find a link to a flash version of the game below the following jolly tune. Enjoy