My favorite? Probably still the original. The first game just has a special place in my heart, tho I concur VC4 has the best overall gameplay. VC2 has that insufferable high school theme, throws in too many mechanics that don't really add much, and is off course badly hamstrung by the limitations of the PSP. VC3 pairs back most of the problems of 2, and is arguably what 2 should've been, but still suffers from the platform it's on. It does have my favorite storyline of the four games. All in all, if I had to rank them, it'd go 1-4-3-2. Never bothered with Revolution.
I wonder if Sega sees much of a future for this franchise. I remember reading that VC4 initially did not sell well at all, tho it apparently eventually eked out 1 mil copies, indicating that much like the first game, it might be the kind of game that relies on the long tail. 1 mil across four platforms is decent for a niche title, but only a little over half the sales of VC1, while afaik the other games did considerably less. From Sega's pov that's probably not a glowing recommendation for continued investment.