I have a nostalgic soft spot for the original Red Alert, but Tiberian Sun has some simply fantastic atmosphere for an RTS of its era, and Red Alert 2 lives as the high point in my eyes. While the core game eventually became purely dedicated to tank spam, the foundation allowed for the excellent Mental Omega mod, which is still in active development. It brings a fourth main faction (the Foehn Revolt, the closest thing a C&C game has ever had to a quality-over-quantity faction), three Generals-style subfactions for each main faction (i.e. the Allies are divided into the USA, Euro Alliance and Pacific Front, each with a number of unique infantry, vehicles, structures and special abilities), new soundtrack (still over 50% Frank Klepacki), a rebuilt launcher that allows for modern resolutions & networking fuctionality. If you're a masochist, it also has a brutally difficult campaign that I've never been able to get into, but the skirmishes are actually super fun and it has a non-dead multiplayer community. The gameplay has specifically been rebalanced to shift toward using more than just swarms of tanks and somehow managed to make infantry completely viable in the RA2 engine.
There's other stuff like Twisted Insurrection, which is a standalone mod (thanks to the base Tiberian Sun game being freeware) that assumes Nod won the First Tiberium War. It has completely reworked GDI and Nod factions with a more accessible and less absurdly difficult campaign than Mental Omega.
It's mods like this that actually keep the spirit of C&C alive and try to bring it into the modern age without compromising a lot of what defined the franchise. EA was entirely too quick to do that with C&C3, which was a decent RTS but gave up on a lot of the Tiberian Sun worldbuilding and is tonally inconsistent with its predecessor. Kane's Wrath did a bit to help with this but couldn't fully bring things back around. Red Alert 3 continued with the absurdity of its predecessors but was mechanically no longer anything approaching the earlier games.
We don't talk about C&C 4.