It's a matter of opinion but give a good reason why.
I can't say because I have only played TD TS RA
But even though I haven't played RA 2 (which I can soon, thanks to CnCNet),out of the three out I have played-
I have reasons for all of them-
Tiberium Dawn-I love the story and futuristic, epic style to the game, the music it awesome too. Its also more balanced than RA in my opinion. Good story, Great Music,Fantastic Multiplayer. Whats not to love?
Tiberium Sun-Although its my least favourite on this list I love the graphics,I love titans and the music is interesting and its a good fun overall but doesn't beat the original,but as I have not played as much as the others I can not praise it as much a it deserves.
Red Alert-If it was down to nostalgia it would be RA that wins instantly ah,the PS1 days the screen zoomed in like a magnifying glass.But RA has great style 1950's cold war.Best music to date in the series for me.Great units.Its a very serious game.Funny cutscenes like TD and overall it just felt right,it was smooth and a solid game.
But I can't decide what it my personal best,I'll edit this post and tell you my answer soon.
Tell me your favourite.
NOTE:Dune series is allowed.