Huh, this thread just seems to keep rearing its head. I suppose having ridden Baron and Valkyria within the last year or so, it's time for me to add my thoughts.
Oblivion
The original, and as far as I'm concerned, still the best. It's simple, intense, delivers exactly what it sets out to do, and no more. That slow turn towards the drop (that no other dive machine duplicates), followed the airtime all the way down the drop, and the tunnel that looks way too small and is entered whilst still almost vertical makes for a ride experience unlike any of the other dive machines that have followed. Sure, more could have been done with the layout, but as we're finding out with the newer dive machines, more doesn't necessarily mean better.
Baron 1898
It's an attractive ride, and has a solid layout. It's not perfectly smooth all the way around (I seem to remember a bit of a jolt in one of the elements), but it's certainly good fun, and the drop is still solid, despite the smaller size. I'm not a fan of the throughput, though - the slow loading, dispatching and offloading annoys me a bit. The station is clearly designed for style over efficiency, when some minor design tweaks could have delivered the latter without compromising the former.
Griffon
A reasonable ride that never really felt like more than the sum of its parts. The front, outside seats are pretty cool, due to how wide the trains are and therefore how far out you are from the track, but otherwise the ride just like the embodiment of modern B&M blandness. Meh.
Valkyria
This is the really disappointing one. The ride layout after the drop is really good. It's really smooth and fun. On paper this should easily be the best of the lot.
What's the problem, then? The stupid vest restraints.
As far as I'm concerned, any dive machine with a poor drop completely fails as a coaster, regardless of what follows. Your mileage may vary, depending on your build, but for me, the vests just hold me down into the seat, and kill any sense of airtime whatsoever going down the drop. It's not the track profile that's the issue, as if hold the vest up against the bars that support the OTSR, there's actual airtime to be had. Given that B&M seem to be using the vests on all their new installations, that probably means that we've already seen the last of the decent dive machines, which is a shame. There's still potential in the ride type, but I now have zero interest in any new dive machines that are announced.