Definitely the graphics engine in F-15 Strike Eagle III was cutting edge at the time. While there were only flight sims that used vectored flat shaded graphics, MPS Labs releases a game with pseudo-textured ground (hills and 3D models were not textured still) amazing cloud layers which are part transparent if you go above them.
I remember seeing the screenshots in some game magazine beck in early '90. I had Amiga 500 back then, and simple shaded horizon would impress me at the time (Amiga flight sims had single coloured sky, drawing shaded horizon which could roll around your cockpit as you flew was too much for ~7MHz Motorola 68000 CPU
). F-15 SEIII blew me away. I think it was the first impulse to switch to PC.