Not super scientific, but I did a quick calculation of how far each flying (or sort-of flying) mount can fly from a mid-ish height. From a high height -- high enough to do a Griffon power dive, with flap -- the Griffin would probably have huge numbers. The ultimate distance is probably using Skyscale to climb a very high object, then Griffon from there.
I went to Hafren on Sandswept Isles and did all test flights off of the cliff, towards the ocean. I used the distance from Hafren in the map view, which is straight-line distance and maybe not as accurate as some measures. Using a little geometry and eyeballing "level" with the Skyscale, I calculate that the top of the cliff about 1650 units above sea level. I started each jump from the second bump at the edge, which is slightly higher, and about 350 units from Hafren.
At the end, when the mount hit the water and dismounted me, I went back to the map view and just used the straight-line distance indicated on the map. I divided that straight-line distance by 1650 (the height of the cliff) to calculate the glide ratio. I should have used more geometry and calculated the sea-surface distance rather than the straight-line distance, but considering how unscientific my experiment was, I figure the ratios are at least comparable:
All approximate glide ratios (distance flown divided by starting height, using the incorrect straight-line rather than sea-level distance):
Raptor leap and fall: 1.7*
Jackal (no boost): 2.5
Glider (no boost): 2.9
Glider (boost W): 3.7
Skimmer (boosted): 3.9
Jackal (Bond of Vigor): 4.6
Griffon (no boost): 4.9
Griffin (flap): 12.8
Griffin (flap & Bond of Vigor): 16.6
Skyscale straight up, then Bond of Faith and Griffin flap and Bond of Vigor: 21.1
Obviously, the Raptor is a one-shot, and the Jackal plus Bond of Vigor isn't very sustainable either. I'd say that Gliding is the baseline, and over a long distance -- from great height or over sufficiently down-sloping terrain -- you can probably achieve 3.2 or so by using the boost as often as it recharges. (In this test, the boost ran out just about as I hit the water.)
For the winning combination: the timing coming out of Bond of Faith into the Griffin is tricky. For all but the last number, we could extend the distance by doing a Bond of Faith just before hitting the water. When I say "Griffin (flap)" I'm talking manually flapping at 70% rather than just holding the space bar down and getting auto-flap at 100%. If we had a ramp, a boosted Roller Beetle would have been competitive with the shorter leaps.
I tried the ultimately complex: Skyscale up, then angle up and double leap (V), then Bond of Vigor, then double leap, then Bond of Faith, then Griffin, but the Bond of Vigor and the leaps put you into height debt and the Skyscale starts sinking while you're trying the next thing, and there's a delay in being able to mount out of Bond of Faith -- maybe you can speed up the process with an intermediate switch to gliding? -- so I never got this to work well. (Plus, you've used the Bond of Vigor on the Skyscale so it's not available for the Griffin and all you have left is wing flaps.)
As a bonus, a couple of tips I should've known about mounts but just found out recently: a) you can mount the Skimmer when you're at the surface of deep water, b) the Jackal ports in the camera direction, so you can do crazy redirects in mid-air, and c) if you ascend with the Skyscale and the top of the cliff is just out of reach (assuming you don't have the third mastery), you can angle the camera up and V and maybe crest the top. (The Raptor and Springer can also be redirected mid-air of course.) Feel free to add your tips in this thread!