Mortal Kombat doesn't count for me, because although I got it when I was 13 it was only rated 15. Similarly, the Doom games were only rated 15 in the UK, so they don't count either.
To be honest, I don't think many games were rated 18 in the UK during the 90's. The first 18 rated game I can remember getting was Carmageddon, which controversially courted a 18 rating from the BBFC, although I was 17 when it was released and I may have actually been 18 when I bought it.
However, if I were to count a game that would have received an M or 18 rating in America if such rating were in place at the time, then it would have probably been one of the more risque game on my ZX Spectrum, one of the games actually aimed at adults with slightly blue humour and sex references, like How To Be A Complete Bastard or Rockstar Ate My Hamster.
Violence wise, the Spectrum ports of Robocop, Renegade or Target: Renegade possibly would have received an 18 rating, since the were pretty visceral despite their lack of sophisticated graphics.