From the eyes of a newer player that joined in BFA and played in classic for some time, I'd say the game is pretty decent. I'm fairly young, so I could be looking at the game through the eyes of that teenage kid most older players were back in 2004-08, but I think the game is worth it either way.
I recommend both the live version and the classic version. The live version if your main interest is just fighting and combat. Raiding and mythic+. The endgame battles of the live game feel like they're generally designed well and pretty challenging.
Classic and TBC classic feel more like a game of exploration. Depending on if you take your time and aren't pressured to reach end-game, it really feels like you grow with your character because leveling takes so long. I haven't really touched raiding on any of my characters because I've been doing next to nothing but listening to the games calming music, fishing, exploring Azeroth, and roleplaying.
However, if you aren't into that, classic/TBC classic still offers the challenge of combat with raids and hard dungeon bosses. I'd definitely give the game a shot if you haven't experienced the game yet.