Of their later works "Fire of unknown Origin" is their masterpiece, every song a winner, of the earlier stuff "Secret treaties", middle (S.E.E. period) period: Spectres, more a sophisticated pop than a hard rock album, but elegant and cool. Revölution by Night, which followed "Fire ...", is a strong follow up (if you ignore the electronic Simmons drums that are all over it, courtesy of their lighting manager suddenly becoming their drummer) though by then they were commercially in decline.
S.E.E has been lovingly remastered a few years ago with a multitude of extra live tracks.