From best to worst:

1985 was the most awesome year. Cobra could've stop right there and be fine, except lacking a basic generic pilot.

1986: For better or worse, redoes the basic ground forces. Adds two pilot classes, plus the so-so Motor Vipers.

1983: Swivel arm Troopers, officers, glider pilots and HISS drivers...I could almost be content with those alone.

1990: Solid bunch of enemy army builders, weakest being SAW-Viper, based on his appearance.

1989: new troops and repaints. Some of are limited use (TARGAT) though.

1987: Techno Viper, Royal Guard (strange but cool) and some nice drivers.

1991: Immortal and Desert Scorpions, plus some other decent troopers

1988: IG's stand out here. The rest are honestly disposable in the big picture, except maybe Toxo Viper.

1992: Flak Viper and Headhunter and Air Devil

1993: Some new molds that deserved better.

1994: Much the same.

1984: Wow...stinger driver.