Now this Is my favorite lesser know RTS game. Developed by Eugen system (Act of War series, Wargame series). The games tells the story of a US major, forced to participate in the galactic arena to earn his freedom. Juicy graphics and FX that somewhat holds event today. 3 factions: Nam' era US soldiers, Cyborg army and generic Bug-like army. All of which has it's own campaign. No base building, instead you earn reinforcement points, which you can use to spawn your troops at special Spawn areas. Can't stop loving this game.

This one is kinda odd but fun. What If scenario for WW2. 3 Factions (USSR, Germany and US). Build your base, construct a massive army and attack your enemy. Classic C&C Formula. What stands out the most - the Weird World War aspect. You got huge assault zeppelins, walking mechs, frost bombs, dual rotor helicopters, that kind of stuff. Red Alert style.

I found it for free on Steam; I didn't really understand the game at all, but I know one of you will get the most out of it and enjoy it :D

As I assume many here, I grew up during late 90s / early 2000s RTS boom.

First strategy game I ever played was the OG AoE demo found in a Win98 disk.

At the time I was playing: AoE, EE, Cossacks, Stronghold, C&C, Imperium, RoN and other "mainstream" ones.

Recently, I came across Warzone2100 and got really into it (it's also FOSS now so that's great)

Any other, less mainstream ones, I might have missed at time, that you guys consider must-plays?

Are Earth2140/2150 and Seven Kingdoms any good?

This one holds special place in my heart. Created by Pandemic Studio (THAT Pandemic studio) in 2002, it offers a unique take on Army Men Series.
You command plastic soldiers, fighting other plastic soldiers in everyday environments, e.g. backyard, kitchen e.t.c. Unit roster is pretty generic: Rifle soldiers, Bazooka soldiers, Tanks, Half Tracks.
Every RTS lover needs to play this at least once.