III is alright. It's the single most visually appealing game in the series and one of the best looking games on the PS2 IMO. Plus it has a ridiculous amount of single player content including two lengthy and completely separate story modes. Seriously, to see the entire game you'll need to put in a good 30+ hours. That being said, it's notoriously unbalanced and had it's share of high-level exploits that nearly broke the game. If you want something casual, this game is great. If you want something competitive, hunt down the significantly rebalanced arcade version.
SCIV is probably the best game in terms of competitive play (even if Hilde was horrible broken at launch, but people found ways around her). It's easily the most balanced game, even the game's PS3 guest character, Darth Vader, was actually viable to an extent. That being said, if you're not interested in competitive play then its pretty underwhelming. The single player content is pretty lackluster and the game falls hard into the "real is brown and gray" traps that so many games felt into at the beginning of the HD era. That aside, it's a pretty solid game and I'd recommend looking into since you can find it for dirt cheap these days.
Now we get to SCV, which was ambitious as all hell but unfortunately their ambitions stretched beyond their budget and time limit. Many mechanics were completely overhauled or removed, meaning that knowledge of the previous games didn't necessarily apply here. However, once again the single player was garbage (at this point I realized Namco thought that custom characters should be enough for single player) and the story mode was short and underwhelming. To be fair, only a third of the planned story mode made it into the game, on top of a ton of other features most of it had to be cut for time related reasons. Anyway, SCV also introduced a ton of new characters, as it was intended to be a semi-reboot via timeskip. Unfortunately, most of these characters were really unappealing and played almost identically to fan favorites that were cut, making many (myself included) wonder what the point of it all ways. Oh yeah, and I'll add that a lot of the characters are incomplete with a few having dead-end move sets.
And that's my opinion on the last three SC games. HOORAY!