Tô me on a personal level, of course, it'll always be Code Veronica. From a gameplay perspective, it ran back to a lot of good things RE3 had bought, like a dodge move, a dedicated button for firing in hazards like explosive barrels and the different branches(it was done by live selection in 3, but it could only be seen as different ways to make the story progress, while in Code Veronica, it's always the same.)
As for the story, it brought a lot of anime bio fantasy into the plot. You can just compare the design of Birkin's monstrous form or Nemesis to Alexia and Miss Ashford feels like she came from an anime instead of a horror movie. Also, it started the trend of cartoonish exaggerated villains, with Alfred and Alexia and then we had original Saddler (I very much prefer his remake incarnation), Salazar, Morpheus, Young Opera singing James Marcus leech, Norman from Revelations, Irving and even Wesker in RE5 feels a little cartoonish in his plan and mannerisms (which could be DC Douglas's way of voicing him though)
And finally, as a fun videogame, I would still put it below or in par with my least favorite games in the series, like zero and 7 (although purely on the fun I have while I play it, I would put CV over 7, but if I would analyze it as a whole, CV does more stuff that I personally don't like)