Looking for a "definitive" version of any Dragon Quest game only leads to sorrow
1: Content wise, all ports/remakes are the same. But... the Switch version, probably the easiest to play, has some weird graphics "improvements"
2: Same as 1, content is the same, preference is just on graphics between mobile and Switch version and if you want to play the game on mobile or not
3: There is no definitive version. All modern ports/remakes are missing content from both the Super Famicom and GBC remakes. The GBC version is feature complete, but it's on the GBC. The Super Famicom remake has arguably the best graphics, but is missing GBC features and was never released outside Japan. The Switch port also has the same weird graphics as 1 and 2
4: DS remake is missing features. Mobile port has these features back, but is "stuck" on mobile. No console release outside of original NES game and Japanese only PS remake (which the DS remake is based off of).
5: PS2 version has arguably the best graphics, but never left Japan. DS remake was, though, and is good. Mobile version is basically the same as DS. No console release for DS version.
6: DS/Mobile remake is missing monster catching, Super Famicom version never left Japan. No other console version.
7: Best version is forever stuck on 3DS, and if it ever leaves the 3DS will be stuck to Nintendo consoles
8: PS2 version is missing features from the 3DS remake, but 3DS remake lacks orchestrated music of the PS2 version and has worse graphics
9: Only ever released on DS
10: Never released outside Japan
11: Switch version has the most features, but graphics aren't as good as PS4/PC version
Basically, if you want a definitive console version of the Dragon Quest series you're... not going to ever find it, other than maybe 10, but that never left Japan!