Hello all I just got the game. I have played diablo 3 but not diablo 1 or 2. So I decided to get diablo 1 since it was on GOG (hopefully GOG can get diablo 2 lol)

Anyway my first question is should I play the original diablo first or should I play diablo with the hellfire expansion?

I started playing the original game thinking hellfire was already in it but looking at the GOG menu Hellfire is it's own thing. So should I continue playing the original or switch over to hellfire?

Second Question do enemies respawn or no?

Diablo 1 is short enough that you could play your first game without Hellfire then a subsequent play-through with it. Hellfire is based on Diablo v1.05 and added new features on top of that. The base game continued getting more patches (up to 1.09) but never included the Hellfire features, and Hellfire never got patches 1.06 - 1.09 -- so they're kind of two separate yet very closely related games. I think it's worth playing both to see the differences.

Enemies do not respawn in single-player. Also, the current dungeon layout remains the same when you load a saved game in single-player. But in multi-player the dungeon will be recreated along with a fresh batch of enemies every time you load a game.

Enemies do not respawn[1]. You may, if you choose, start a New Game, and in that new game, there will be a new randomly generated set of dungeon levels, and those levels will be populated with a new randomly picked (within limits) set of enemies. For the scope of your question, enemies will not respawn as a result of you lingering, either on the level or anywhere else. However, enemies which have not been killed will regenerate over time, and eventually return to full health. Therefore, you should make it a point to promptly kill anything you have wounded.

[1] Due to a bug, enemies killed by a trap in multiplayer may revert to life when you reenter the level.

Diablo has 16 floors of increasingly difficult (on average) monsters. Hellfire adds another 8 floors, though they are not strictly sequenced relative to the Diablo floors.

If you play in multiplayer (whether or not anyone else joins), you can use New Game at any time to get a new dungeon with fresh enemies. Additionally, in multiplayer, you have direct access to the deeper floors (5, 9, 13) once you reach the appropriate character level. This makes it easy to play just one area, such as floors 13-16, quit, and do just those floors again. This will provide a similar experience to enemies respawning.

Respawning enemies seams strange to me, it feels like bit games where there wasn't enough memory to remember the enemy state

So you can keep playing multiplayer over again by yourself? To extend your characters playtime?

Correct. When you defeat the final boss on level 16, everyone on level 16 will automatically leave the game, whether they want to or not. Be sure to pick up anything you want to keep before you finish that fight. If there is another player in the game, who is not on level 16 at the time of the boss's death, then that player can hold the game open and others can rejoin. If every player was on level 16, the game will close and all items on the ground will be lost.

Whether you leave the game because the game forced you out due to the death of the boss or because you chose New Game from the menu, your multiplayer character will retain his/her spells, items (including backpack), and statistics. Any items on the ground are lost. You may then create a new game, with a new dungeon and new enemies, and continue playing with that character. You may do this as many times as you wish, and it is how characters achieve the maximum character level (50). There is nowhere close to enough experience in a single game to achieve that. An advanced character, operated by a skilled player, can clear the deep floors several times in one multi-hour sitting.

Note that multiplayer does not have a specific save function, so you must use New Game (or Quit, which has the same effect) when you are done for the day and want to use your computer for something else. This puts a tight limit on how much equipment you can preserve long term (and led to the rise of Ulmo's Stash, among other third-party solutions).

When a character is killed in multiplayer, you do not reload from a saved game. Instead, you restart in town, and your equipped items (and half your carried gold) are left at the spot where you died.[1] You must then fight your way back to that site and recover your equipment. If you leave the game before then, your equipment will be lost.

[1] In multiplayer, dying on level 16 does not drop anything, because of the risk that you would end up losing your gear due to the death of the boss while you are trying to get to your death site.

No offense but I like the sound of multiplayer more lol. They should of had a new game option in story mode truthfully. Multiplayer sounds like it's the meat of the game truthfully.

Also I downloaded the hellfire expanded mod. So it added multiplayer to that to lol.

The ability to start a new game with you current character is available in single player mode too.

Multiplayer mode, whether alone or with others, is more popular with people who wanted to get the most out of the game.

Single player also has the New Game option, which works mostly the same. However, in single player, the deeper areas are only opened once you have reached them normally, so you must fight your way through the early floors again even if you only want to see the deep floors. Most people don't like that, which is why multiplayer is preferred.

Note that multiplayer has even less story than single player. There are only 4 quests, and they are always enabled: The Butcher (floor 2), The Skeleton King (floor 3, randomly placed; unlike SP, where this takes place in a special mini-level), The Vile Betrayer (floor 15, fixed location; not on a mini-level), and Diablo (floor 16; whole floor's layout is fixed).

In short:
you should play from Hellfire v1.01 exe (DX GOG or original) if you want latest version with all content.

Original Diablo - some retail version even without patches. Just for fun to compare differences what game be back then before updates and Hellfire.
But for complete walkthrough - Hellfire, cause it build-in add-on with two new levels and one new hero type.

Its too much posts in this such easy-to-answer topic. Some players didnt even play Hellfire and dont know what it is. It still original Diablo. Im not talk about posts above - it from Blizzard forums or some unofficial.

As Ryan333 noted above, Diablo continued receiving bug fixes after Hellfire was released. Hellfire has more content, but Diablo has fewer bugs. Arguably, Diablo is the "latest version" since it continued to receive updates well after Hellfire v1.01 was released.

This thread reached its current length because the original poster continued to ask questions, and people posted answers to those questions, as well as some information that it seemed like the OP would want to know.

Of course, at the complication of using third party content, DevilutionX might be the best answer - especially once its Hellfire support is finalized. As I understand it, DevilutionX is an improvement over Diablo in a variety of ways.

Soon you will be able to play hellfire with 1.09 bugfixes thanks to https://github.com/diasurgical/devilutionX : P

But mods\remakes =\= original game. It always only alternative.