I'd say Warcraft II: The Tides of Darkness

For me it was RA 3. I did enjoy C & C 3 t tried to be faithfull to the prior games and was a lot of fun. C & C 4 shold not have existed it was a trainwreck and was no C & C game.

Just wish the remasters of red alert etc come here i would settle for playing them again just for anenhanced music as you cant beat Hell March.

C&C4 was the game that ruined it for me. Then Rivals came along and made things worse with all the microtransaction crap.
If the remasters turn out to be as good as the information so far suggests they will be, I will be very happy and will definitely be adding them to my collection.

As for playing the older games on modern OSs, I have The Ultimate Collection on Origin and all the games worked just fine on Windows 7 last time I tried them (including the originals). And there are options for playing a few of the games that improve them beyond the originals in terms of compatibility etc (e.g. the Tiberian Technologies patch for C&C Renegade of which I am the lead developer)

RA3 was a different beast, but I've enjoyed it a lot. While everything after it were utter garbage

C&C 4.

The infection started with Tiberium (why not Tiberian?) Sun, we luckily got some good games but then came Generals and then Red Alert 3. C&C4 was the bullet to the head.