I'm a relative newcomer to the C&C franchise. The only game from the series I knew and loved was Red Alert 2, and then I played C&C3 for a while, but left it unfinished (for reasons I can't remember now). Then, I got Tiberian Sun and Red Alert 1, and after that C&C: The First Decade. I had the chance to play all titles from the series from before C&C3, and did that, with only Generals left unplayed at the moment (will fix that, just don't know when). So my love for the series is pretty fresh, only about a year and a half old.
That said, I consider C&C3 to be the best C&C installment, in the same way I think RA2 is the best Red Alert. It's got all the things I thought it needed - beautiful graphics, interesting units, good plot and great in-game movies. Though the game gets me with the same stuff as any part of the franchise (I don't really know how to use the units I get efficiently), I still enjoyed the whole campaign experience (I don't really do Skirmish games with RTS').
There's one issue that bugged me a lot though. I started playing the GDI campaign on the Normal difficulty, and faired pretty well up until a certain mission, when I had my ass handed to me with little effort on the part of the AI. I changed the difficulty to Easy... and experienced no difference at all. When I finally managed to pass the mission, I decided to play on Easy all the time now, to save myself the stress and curse words. And see, it's not that the game was difficult - it was absolutely unfair. How was I supposed to retrieve my MCV, while at the same time I had to defend my base from all four sides? The AI didn't have to mine for minerals to dish out tens of units each minute, while I had to fight for every tiberium crystal I got, and the moment I got it I had to use it and then get some more, because repairing the base costs cash.
I've experienced the same scenario several times during the campaign. I know that plot-wise, when you fight on enemy ground they will probably have infinite resources compared to you, but it's still friggin unfair that you're under attack 24/7 and whatever you do, you'll still be attacked relentlessly. This was especially irritating in the first mission of the third campaign, when you had to use crappy units against a well organised and far more experienced enemy force which attacks you ALL THE TIME, with little to no pauses. Not cool, people. And that was EASY difficulty, though for me it was more like Hard.
If not for this issue, the unfair and absolutely random (some Easy missions were banal, some others were almost impossible to do) difficulty level of the campaign missions, this would be a strong 9/10. But for me, the single player campaign is the only thing I get out of an RTS, which means it's the single most important thing. Which is why C&C3 has to get 8/10 on my scale. I just installed Kane's Wrath, and I'm just hoping it's not going to be as irritating.