Sega Master System / Mark III / Game Gear
This elderly shoot 'em up gives you control of a Transformer-style robot who is swooping over alien landscapes, zapping away at all sorts of alien ships. Killed ships leave behind letters which provide alternative weapons systems for our plucky automaton, making his horrendous task a little easier to cope with.
This earty Sega shooter was a bit basic compared to more recent titles such as R-Type, but at least there are extra weapons to keep a bit of variety. The other plus is that now it sells in Sega's ten quid range, so it's well worth rooting out if you're low on moolah.