I just spent about 30 minutes testing everything, and it would appear that-
Airdodge > doublejump > smash forward B
is the best way to momentum cancel if you're dying horizontally.
Vertically, it's still just bair or dair, as you're able to fastfall while performing your aerial.
Yeah that makes sense, but you should usually DI to the top corners of the blastzone, because it's the most far away point from your position... and in this case it's best to bair & fastfall & DJ (& sideB).
I don't get how dair would be better than bair+fastfall, because from my testing it isn't. It doesn't have any special properties, it just fastfalls automatically and the people that found out it "momentum cancels better" probably just never fastfalled the other aerials or something... <_<
But I noticed something... if you get hit into a top corner and bair +fastfall then double jump I died immediately after using the DJ, but when I waited a little bit before using the DJ I survived (without the double jump and only a dair or bair+fastfall I died). I also survived using fair(&fastfall) and then DJ, you just have a wait a little bit before double jumping... the thing with dair is, it has so much ending lag that if you DJ after it immediately you'll survive.
And you only die from bair+DJ, because you DJ too early.
I could also survive with airdodge+fastfall and dj, but I also had to wait a little bit before DJing or else I died.
(this testing was done at 160 % (middle of FD, MK did dsmash on Diddy (training mode)); I ALWAYS held up on the controlstick while getting hit)
At a bit later % (165)... you can't survive with airdodge then fastfall and DJ. No matter what I did I always died.
I could survive with
dair+dj and
bair+fastfall+wait a bit + dj and with
bair+fastfall+sideB.
At 170 % I couldn't survive with dair because it has too much ending lag. But I could survive with bair+fastfall+waitjustalittlebit+sideB (DJ KOd me). It was also possible to survive with fair+fastfall+DJ+sideB...
172 % I could also survive with bair+fastfall+waitjustalittlebit+sideB, fair had too much ending lag.