They've all got different appeals, to be honest. I haven't flipped in the traditional sense with MDMA or a proper entactogen but I have done my fair share of disso + psych flipping, particularly LSD + DXM, mushrooms + DXM, and then more recently ketamine and 4-pro-dmt (vaped).
LSD+DXM: Most euphoric out of what I listed, definitely the strongest if youre just looking at moderate doses of each, and definitely the best in terms of visuals. I bet ketamine and LSD are great together (hopefully I'll try it soon...) but something about the peculiar nature of the DXM headspace and trip, it makes the entire experience very alien and strange. Like, truly alien, seeing languages that don't exist sprawl all across your walls and being so high you listen to the same album 12 full times in a row because it sounds like heaven.
Speaking ideally, I bet the old ketamine + MDMA + LSD trips are amazing, but could probably be improved upon. I personally really prefer 5-MAPB over MDMA as it feels much more cleaner and more consistent, definitely more forgiving too and I dont like dealing with a roll comedown while i'm also wiped out from a psychedelic, 5-MAPB just sort of ends and then gives you a bit of an afterglow and then you're back to normal. Plus, ketamine is a bit weak and not as trippy or fun as it could be. I think based on my experiences that FXE would mix reallllly well with a psychedelic or a flip but of course it has more room for things like mania and its more stimulating so that would be another reason to go for 5-MAPB over MDMA as I found it to be in general less physically stimulating. As for the psychedelic... LSD is always probably going to be the classic choice here but I think some of the 4-ho tryptamines that have been available are AMAZING and so well suited for mixing with other substances due to their lack of challenging headspace and lack of body load. 4-HO-MET, 4-HO-DET, and 4-HO-MIPT are all fantastic choices for a flip but I think a mix of 4-HO-MET + 4-HO-DET is ideal for the psychedelic component as the -MET gives you the super clean strong psychedelia and eye candy, and -DET gives you the delicious psychedelic body high and rounds out the sort of semi-hollowness of the "trippiness" of the 4-HO-MET experience.
So all in all, I think ideally it would look something like this:
80-400+ mg of FXE (split across entire night)
75mg 5-MAPB
7.5 - 20mg 4-HO-MET
7.5 - 20mg 4-HO-DET (equal dose to 4-HO-MET)
Optional: Cannabis
A couple bumps of FXE or a similar arylcyclohexylamine of your preference to get the brain going and to give yourself sort of a shield for the come-up if you're prone to feeling weird on the comeup. Tho this is optional bc if you're following this plan your come up will be super smooth and not unpleasant
Then whenever you feel settled, take 75mg 5-MAPB and anywhere from 7.5 to 20mg of 4-HO-MET, as well as an equal dose of 7.5 to 20mg 4-HO-DET. The even ratio is something I've tried a couple times and it works amazingly, produces very recreational fun engaging trips and the DET gives me such intense, fun giggles and makes stuff so funny... they really round each other out so well.
And then for the next 50-90 minutes as you come up on the pseudocandyflip, refrain from any FXE or cannabis so that you can adjust smoothly to the new chemicals in your system and get yourself settled to further dose the other substances.
Once you're feeling yourself get to the point where it feels right, resume use of the FXE. I think ideally, IN is best for this setting, as it allows for continual, gradual, incremenetal dosing. I like plugging and some people really like oral dissos but I think I'd just stick to snorting. Once the FXE gets you to a point where you feel satisfied, if you feel like you need any more intensity or anything to add to the experience you can smoke some weed. Maybe this weed option would be something preprepared like a vape, a joint, or a pre-loaded bong hit or pipe or something. Though honestly I dont recommend adding weed on top of this pile of confusion, some people will just do it regardless (like me) so this seems like the absolute best and most pleasant way to add it to this optimized flip