Sinclair Spectrum for me definately. I started out with a 16K rubber key beer mat speccy then went to a 48K rubber key, then sent that sway to be upgraded to a 48K+ then the 128K then plus 3 model. I had a multiface 3 whch was pretty awesome that allowed ou to dump ram contents to disk (save states).
The spectrum conversion of renegade is just amazing on the spectrum, great colour, great sound and still fun to play even today, compare it to the C64 version.
The C64 however was a competant computer and it's SID chip in the hands of the right people could do some amazing sounds, the Amstrad seemed to be a curio owned by about 2 people at school.
Some stand out games for the sytem for me personally were...
Knight Lore - Hard to imagine what a groundbreaking this game was.
Mercenary - Pure witchcraft, you could fly around a wireframe world even even navigate corridors underground.
Driller / Castle master - Sure it was really slow but it blows my mind that this was even possible to have a full 3D multi room world on a 48K system.
Dizzy Series: These always seemed best on the spectrum to be, I had them on the Amiga later on and they were nowhere near as good.
Honorable mentions
Brian Bloodaxe, Alchemist, Rapscallion, Caves of Doom, midnight resistance