Would be interested to hear on what RTS people think is the best on GOG and why? Looking for an RTS to try.

Total Annihilation, KKND, Pax Imperia 2 and Nexust the Jupiter Incident are very good just to name a few.

That's probably impossible to answer since you have a lot of different RTS sugenres, eras, or epoch to play in.

If you loved Age of Empires from Microsoft you have: Star Wars Galactic Battlegrounds saga

If you like massive battles on an epic scale you have: Total Annihilation

If you like C&C Red Alert you have Act of War Gold Edition which is very much similar while still different enough.

If you like a RTS in medieval setting you have Firefly's classic Stronghold HD

For some epic 18th century fights there is Cossacks Anthology

For some epic sci-fi enspired RTS/4x mix up you have Arcen games: AI WAR

Earth 2160 as a very good apocalypse like RTS: http://www.gog.com/game/earth_2160

Perimeter with a very unique defense/offense system (aka controlable shield wall): http://www.gog.com/game/perimeter

Sins of a Solar Empire for epic space oriented RTS: http://www.gog.com/game/sins_of_a_solar_empire_rebellion_ultimate_edition

There a lot of others to choose from but impossible to pick one out.

Just a small note - Nexus Jupiter Incident is not an RTS. Its a pure RTT game.

Red Alert 2 ofc

Thanks for the recommendations! Stronghold one looks good. Also just remembered I have C&C3 Tiberium Wars that I haven't played in years!

From GOG ? Sacrifice, Impossible Creatures,Blitzkrieg, Divinity: Dragon Commander and the Warlords Battlecry series.

These aren't pure RTS's.

In addition to the ones mentined above there is Settlers 2 Gold which is a mix of RTS and management game.

I also heard Battle Realms is pretty good but I haven't tried it myself. It was given away for free some time ago.

Battle Realms is definitely very good with the japanese/chinese fantasy setting. Settlers 2 definitely an epic game but I don't know. Never could see it as an rts more of a simulation which is why I didn't list here.

RTS is pretty broad so it's difficult to pick a favorite. If we narrow it down a bit though:

Going for the more traditional style of rts, with trivial resource gathering and basebuilding:
KKND
Dark Reign
Both are great fun, but quite difficult, especially the latter.

Straying a bit from the more traditional feel by adding some nontrivial base management, there is the excellent
Stronghold

Starying further away still from the traditional rts formula, there is the timeless and utterly fantastic
Populous

Going the tactics route, with all base building elements removed there is the 'realistic'
World in Conflict
and the futuristic
Ground Control and Ground Control 2

Finally, if you can't go wrong with [url=https://en.wikipedia.org/wiki/Command_%26_Conquer:_Tiberian_Sun]Tiberian Sun[/url] which has been free since 2010.
Another game you could consider is Battlezone which, although played from first person perspective, has rts base building elements (placing refineries, turrets etc.)

I know. But if he likes normal RTS games, he'll like these too.

Thinking about best RTSes, I have recently nominated Total Annihilation to be inducted into the The Strong's World Video Game Hall of Fame (which was mentioned on Gamespot recently).

Heck, I know several other RTSes that should also be inducted (especially the ones from the 90s and possibly 2000s). Y'all can in fact nominate any kind of game that has had a huge impact on the computer game/video game industry.

Link: http://www.museumofplay.org/about/world-video-game-hall-fame

I think they fit the acronym. I wouldn't use the word 'pure', but maybe they aren't typical.
Warlords Battlecry is king, though :p
Another not so typical RTS would be the Spellforce series.

I'm a huge fan of Empire Earth. The first one is the best.

Stronghold is the most "realistic" I've ever played.

And European Escalation had the neatest scale and was quite realistic with its units.