C&C 1 and RA 1 are classics. Clunky in some of the design but overall still very playable and enjoyable. TS and RA 2 are the cream of the crop for both respective universes. Both eliminated the clunkiness of the previous games and flesh everything out much, much more.
Renegade, while not an RTS, I found bland and uninspiring as far as FPS go. The MP was fun for a short while, and it's cool to see everything from ground level, but the novelty wears thin quickly and it's completely forgettable.
Generals is enjoyable if you like that sort of setting. It's modern warfare but with the signature C&C twist of unique units and sides with their own personalities. Not bad, just different.
C&C 3 is a decent continuation of the Tiberium storyline. I dislike some of the game design decisions, but overall it's enjoyable and should be played considering it was the last good Tiberium based C&C game. RA 3 is decent but has a heavy emphasis on co-op, which can hinder the experience a bit. However this is without a doubt, the LAST decent C&C game I'd say is worth playing.
C&C 4 is utter trash, and the nail in the coffin for the series. Let me tell you something, there are a few games I follow very, very closely and love dearly with all my heart. Games that I grew up with, and games that grew up with me. Since the day me and my dad got the first one back in the mid-90's, every few years it was always a joy to keep up with the new C&C game coming out. Getting hyped up for it, seeing the screenshots, reading the reviews... C&C was and is a series of games I remain loyal to, to this day (thanks to awesome ports like OpenRA)
When I played C&C 4 and completed it, I was filled with actual rage. It ruined my whole mood for the rest of the day. The gameplay, the story... what they did to Kane.... Just avoid it. Never play it. Don't even give it the time of day or even the thought. It doesn't exist. That is what you call, a rape of a beloved series of games and a giant fuck you to the loyal fanbase. If you ever come across a physical copy, break it.
tl;dr play all of them except 4 and maybe Renegade since it's so forgettable.
tinyE: I don't think I'd hate #3 so much if I could get the fucking thing to run on my computer. :P
WTF is "Exception in Exception Handler" anyway? :P
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