Wow, Ruliweb is still going, eh? I used to go there all the time back in the early days.
SNES Valis IV is a decent intro to the series I’d say, though it’s not the absolute pinnacle. The PCE Valis games are the best examples, and Super Valis IV is basically a remix of PCE Valis IV in a way I find really interesting. But if you don’t like Super Valis IV there’s a high probability you won’t like the rest of the series, because it isn’t “good” in the traditional sense. You’ve got to be ready for a stilted experience with cool music and obtuse powerups and a weird but vital slide move (super valis iv exchanges that for a running jump).
In terms of ranking I’d put all the PC games at the top (probably 3 is best), followed by Super Valis IV, followed by all the genesis games, with SD valis at the bottom. I haven’t played the MSX original, so I can’t really rank that one.