You can try deleting DDraw.dll from the game. It helps me with alt+tab in Belzebub.
BZW, original Diablo.exe does not work without this dll, if you care

Dead link .

I am having issues with my character's progress not saving. I leveled to 18 and logged out only to discover my progress was not saved and wont save. Running as Admin only got me as far as keeping a character in the menu. Any progress I make is reset each time I log into my character.

What I find especially interesting about Belzebub is, that all the new quests have full voice overed dialogue from every character in town with the obviously original voice actors, is all of that cut content that was restored or did they hire people for that?
It's quite a lot of dialogue if you talk about all the new quests to every character in town, so I wonder.

the belzebub mod was never completed.

i'd check out the hell 2 on moddb.com

it is still undergoing very active development, has an active discord community, has numerous qol improvements, boots, belts, and gloves, improved soundtrack, and all the helfire dungeon levels. all thats needed to run it is the diabdat.mpq file from the original game

https://www.moddb.com/mods/diablo-the-hell-2

Doesn't seem to support widescreen/HD looking at the screenshots although the discription mentions it.
Is it just streched or actual high res with far sight like in Belzebub?

TH2 supports widescreen, but can't zoom in like Belzebub. still under development too.

there's a config.ini folder that allows for changes to resolution

Can't you change window size in options? (O_o)
Press ESC when ingame -> Video options -> Resolution -> (set 1024x768, etc). The window will not be small, of cause

The mod works fine without any problems :)
I'm playing actuelly in 1600x900 (fullscreen) and i can change to any resolution given via the options while playing.
Briliant work from the modder !

Reading through the details of various mods, I'm not sure what to do for the best really.

Belzebub looks great and I have used it previously (got down a few floors of the dungeon and got to a level 8 character) but it does change the gameplay somewhat and tweaks certain elements, such as The Butcher mission.

Infernity seems quite vanilla what with it appearing to only really be adding bug fixes and quality of life improvements, but it is unfinished.

Then we have The Hell (1 and 2) which seems to add a ton of stuff.

So which would be best to use, or should the GOG DX version be considered as a valid alternative here? If The Hell allows a vanilla-like playthrough with all the added extra elements being optional (like the Hellfire dungeons and additional quests) other than bug fixes and quality of life improvements then I may consider it, otherwise the best option for a vanilla-like playthrough would seem to be Infernity, or GOG DX.

Imho they really should have contacted those modders for a better release here, if they didn't do that (they probably didn't), it was a wasted opportunity.
Imagine vanilla with the HD resolution options and only some (necessary) bug fixes and the usual game-play fixes from the better mods (ground searching via ALT, running in town, refreshing mana at Adria, proper stash, waypoints etc...).

Personally, having played vanilla Diablo 1 to completion several times, I'm finding Belzebub (aka Diablo 1 HD Mod) to be quite enjoyable. While it obviously is different from vanilla in significant ways, it feels faithful in spirit to the original Diablo 1 and a pretty good re-imagining in light of Diablo 2.

If you want a pure vanilla experience with QoL improvements, it sounds like this is the way to go:
https://www.gog.com/forum/diablo/diablo_hellfire_with_some_quality_of_life_changes_belixs_hellfire_v102a5_mod

Otherwise, between the various other options, my vote is for Belzebub.

*facepalm* XD
Thank you. :)
Sometimes the simplest solution is the easiest to overlook.