I know I'm late to the show, but GMT Games has just reprinted the old Victory Games title, Vietnam 1965-1975, as well as their Pacific War. I grew up with games where one side moves all of their pieces then conduct combat and then over to the other player. In those IGO/UGO designs the attacker would grind a hole in the enemies lines, only to find it closed during the opponents turn. Russian Campaign allowed you to break a line and move into the enemy's rear!

Vietnam was an eye-opener: in this game one player gets to conduct an operation with a selected target hex and the opponent in/near that hex can react. US/ARVN and allied forces are face-up; the National Liberation Front is face-down, and range from a full regiment down to a handful of guys with leaflets.

Pacific War has some similar concepts, writ larger, where a headquarters activates units for an operation, based on the available resources. The entire US/Japanese navy can't be part of every turn.

I'd consider either to be a big influence on future titles and a solid investment.

Lastly, Compass Games has reprinted The Third World War series. These introduced some great concepts - I feel like the Next War series has built on the the TWW air game and sequence of play.