In my opinion it was Dark Forces:
- a more advanced engine supporting room-over-room, limited look up/down and polygonal models (e.g. Kyle's ship at the start and/or end of missions)
- alternate fire and new types of weapons: hand grenades, grenade launcher (aka mortar)
- larger levels
- puzzles
- plot
All of that came at the cost of higher system requirements (8 MB and 486 CPU IIRC).
Cons:
- no saves during missions
- no multiplayer
- a more advanced engine supporting room-over-room, limited look up/down and polygonal models (e.g. Kyle's ship at the start and/or end of missions)
- alternate fire and new types of weapons: hand grenades, grenade launcher (aka mortar)
- larger levels
- puzzles
- plot
All of that came at the cost of higher system requirements (8 MB and 486 CPU IIRC).
Cons:
- no saves during missions
- no multiplayer