Have to mention this old classic; Total Annihilation (and expansion Core Contingency). Has much similarities to Red Alert and Starcraft...released slightly after it same year. If i could point out some good things about it:
- Unlimited units (if you can afford them by resources)
- Stationary very long range turrets of different kinds (like nuke silos, sniper cannons and massive miniguns that devastate entire bases if built close enough with proper defence)
- Little more advanced AI orders (air units can for example rest on ground but lift off on enemy sight, rapidly engage it and soon return)
- 3D polygonal units
- What makes skirmish fun is that you can tease CPU as long as you like, they will fully recover as long as they can get their hands on resources and any needed construction unit
- Ability to queue different things like constructions, movement, repair...
- Fans are still around modding and playing it online.

But in my long path i've never come across RTS better than Starcraft Tried a lot.