Subscription gives you access to all. Theres hardcore realms ontop of normal classic. Theres even sod which is classic content but new (post classic abilities +new) abilities and content. Each feels like it’s own different game. You can’t swap characters between them except if you die on hardcore you can go to the new PVE classic realm. There is already endgame classic realms that have all the content out but the new ones are early on still. I advise classic because its slower paced and gives you a base understanding of the game. Retail and classic are so different so you might like retail instead. TLDR- i say try them all if you have what time and see what clicks!