Hard to say...some I like a lot though.
Axis and Allies dice combat system - This is actually similar to many wargames, not just A&A, but A&A is the most famous. Many others use similar systems (original Conquest of the Empire, many other wargames, War of the Ring, etc).
Heroquest Dice combat system - This one is also utilized by many these days. I can't recall of a pre-cursor to HQ for this one, so it may have been the first to use it. It uses symbols to represent if you hit, if you stop a hit, and similarly for the enemy dice. Descent used a variation of this for example.
The orginal D&D game system - This would extend to BX, BECMI, and AD&D 1e and 2e. It is more complex than the "D20" system that came after, but I like the variation. It gives it a flavor that really isn't replicated by anything else.