Super Darius II is by far my favorite and the most fun. It's also pretty cheap today.
You should still buy the Saturn port as well, but it is far from arcade perfect.
The SMS version is also a great fun game that everyone should play.
I haven't played Sagaia, but everything I've read on the matter suggests that Sagaia plays closest to the arcade. Super Darius II is a pretty cool game, but the zoomed-in perspective means it actually plays quite differently from Sagaia or Darius II arcade. It might be worth getting simply because it has such a different feel. For all the great track remixes, though, I will say SDII has crappy sound effects. Really oopsed on that one, Taito did.
SDII is rebalanced, unlike Super Darius/ Plus, all of which are made by
NEC Ave. Sagaia/Darius II for MD is also rebalanced to better suit the single screen, but there isn't a definitive arcade version, as it was released in 2 and 3 screen modes. SDII does so much more than simply try to make DII work within a single screen. It's pretty much an original new Darius game and the "zoomed-in" perspective is actually zoomed out compared to the arcade versions. Just as games like Air Zonk play fine with large sprites, so does SDII, as the game was designed around exactly the balance the developer struck.