Ah Space Invaders, perhaps one of the most influential games of all time. Despite this the humble Spectrum didn't receive an official version until 1991 in the form of Super Space Invaders which, is an update to the original game (and also released on other platforms) and required a 128K Speccy.
Take a look on World of Spectrum and there are plenty of unoffical clones, I've picked a couple and along with Super Space Invaders I shall take a look at them now! The interesting thing about the two clones are they both only require a 16K Spectrum.
The first clone I grabbed is Space Intruders, released way back in 1982 by Quicksilva Ltd. This one is the most faithful to the arcade version - its fast and fun, sound is simple but not at all offensive. Does the job nicely, and even has a demo mode which considering its 16K requirements is impressive!
Next up is Base Invaders, released in 1983 by Work Force - again only needing a 16K Speccy! This one allows you choose a speed between 0 and 2 and has the addition of a shield that you can use to protect yourself. The sprites on this one are bigger, but the game play is slower. Sound isn't too bad except when it tries to play a tune at you. All in all a decent effort.
And finally, on to the main event! Super Space Invaders, released in 1991 by Domark.. Finally a licensed Space Invaders on the Sinclair Spectrum! This version was also released on several other 8 and 16 bit platforms. And? Well, it has nice menu music, 2 player mode.. And that's about it - you can't even redefine the keys! The worst thing is it is so damn slow! Until you get down to having one alien left then it goes at stupid speed making getting the little bugger near impossible!
Considering it required a 128K it is very disappointing, I have also played the Master System port and it is a lot better.
So, all things considered I have to say the best version by a long shot is also the eldest! Space Intruders is, in my opinion, the best of the 3 - that's not say there aren't better ones out there, these are just the ones I picked randomly.