That’s a loaded question, but I’ll do my best since I’ve played them all.

Classic - the game as it was right before the first expansion. It’s progression is slow, but meaningful giving players time to feel out their classes and experience all the world building that went into the game. One big downside is that if you don’t play with at least the Quest is add-on, you may get very lost with many quests. They didn’t show where you are supposed to go most of the time, you had to get that info by reading the quest log or items they gave you, or NPC dialogue. Also, dungeons didn’t have detailed maps to look at so you’ll get lost without a guide. However, its my favorite way to play currently because of how it feels and the fact that no updates means knowledge you learn doesn’t go away in a few patches.

Classic Hardcore - when your character dies, that’s it. Gotta make a new one. Super fun way to play because it keeps a healthy mix of player levels as well as making sure gold prices don’t skyrocket since even gold and inventory is lost on death. Wouldn’t recommend it first try but you do you.

Season of Discovery - Imagine classic, but with a whole bunch of new stuff. New abilities to try, new dungeons and raids, new gear. Its still gets fresh content too. It has a similar problem that retail has, where the goal for new players is to just level up as quickly as possible. They even gave every character a buff to give you more exp and gold to hurry you along. The goal here is the newest freshest content.

Cataclysm Classic - after classic was released. Blizzard decided to re release all their expansions in order all over again. Mist of Pandaria is the next expansion coming soon. Not up my alley, but it is a decent snapshot into the past releases. Well except the added micro transactions.

Retail (War within) - wheras the other versions only require you to pay the monthly sub, the freshest expansions need buying to play as well. Retail suffers from a “hurry up and level” syndrome. You don’t get much time to learn your class because levels happen so fast and their put onto your screen immediately. Any gear you get is short lived as you’ll find an upgrade in the next few minutes. Once you get to the max level is where everyone else is at and you can start playing the content that’s most fun. I do like it at times. High tiered dungeons are a lot of fun as it pushes parties to play better and better. Plus you can farm enough gold through play to buy the monthly sub if you’re dedicated enough. The servers are packed as well. Probably more people playing retail than any other version of wow.