I don't have a lot of experience with fiends so far in 5e, but I have a lot from prior editions and Pathfinder both as a DM and as a player of demon hunter concept characters.
I really liked the 1e MM, Fiend Folio, and MMII fiends. Jim Holloway's art in MMII was particularly evocative. I have always found the fiend lords engaging from Demogorgon's entry in the 1e MM on.
I hated fiend gating in 1e-3e. And teleporting. 3e in particular had a lot of fiend teleporting abilities that made them annoying hit and run combatants and drove some stuff towards tactics to magically pin them down which was not quite the combat feel I was usually going for.
Role Aids from Mayfair Games had a nice series of Demons supplements for 1e AD&D that I enjoyed.
2e I really liked the fleshed out characterizations and introduction of the Blood War in the Outer Planes Monstrous Compendium Appendix and then later stuff on fiends in Planescape (which I only came to much later). The Monster Mythology book had a few entries for Demon Lords that were a bunch of fun, including adding a reptilian one as the patron of Lizard Kings. The Book of Hell was a different 2e take that really reconceptualized Asmodeus in particular.
3e was interesting in their take on fiends. I particularly liked mechanical alignment descriptors for fiends and alignment based DR in 3.5. 3.0 had them with a lot of spell-like abilities and SR and such balanced against normal combat stats which made them fairly glass canons for their CR, while 3.5 took away a lot of spell stuff and bumped up their HD and combat stats. Pathfinder continued that trend bumping up their combat stats even more for their CRs.
3e went in pretty hard on the fiend lords not being gods in Book of Vile Darkness which went against the 3e trend of accommodating godless clerics that continued from 2e. It pegged the Demon Lords as low 20s for CRs and was fun to get them back in the game concretely.
Dragon Magazine's 3.5 Demonomicon articles were a big expansion of lore on various demon lords and added a bunch of fun material. It also gave options to bump them up to the high 30s in CR. The fiendish Codexes were great, adding in more concentrated lore and demons and demon lord stats between the BoVD and the Dragon stats. With the addition of Aspects providing low CR avatar type versions of demon lords (like around CR 10 or whatever) there were plenty of options for demon lord stat models at whatever level you liked.
3e also had the explosion of the OGL with gems like Book of Fiends (a lot of cool new fiends and fiend lords, though a lot of underpowered ones that were statted out) and things like the Slayer's Guide to Fiends which had themed demon family types besides the Tanari.
Pathfinder took over the 3e D&D legacy and really expanded the fiends with multiple new group types (Kytons became their own thing, Oni, Deva, Rakshasa) all with their own themes and lords.
4e redid the demon devil cosmology from 2e's souls of the damned in the outer planes and turned all devils into fallen angels and all demons into corrupted elementals which worked well for me narratively. Succubi switched from demons to devils, then later some went back to demons with Grazz'ts new story of being a demon lord who was a corrupted Arch-Devil who was a fallen angel. Mechanically fiends were 4e monsters so each had their own mechanical and role based shticks but not the classic teleport and list of spell like abilities. I enjoyed them as monsters in fights. 4e also had a great Demonomicon book.
5e went back to a lot of 3e fiend lore but also turned Succubi from 0e-3e demons and 4e devils to NE fiends of their own type.
5e has come out with a bunch of Demon Lords in the mega module and some description of fiend lore in Mordenkainen's, but I don't have the module or the monster lore book.
