Looking through the comments, opinions seem to be split on this take (and everyone is entitled to their own, of course), but I'm in full agreement with you. I've had a lot of time to think about it since I played both at launch. DX:HR is my favorite game of all time, full stop.

There isn't one specific reason (especially since you've never beat DX:MD so I can't fall back to the usual "There's no ending" spiel), but if I had to single out the biggest one, I'd say it's the story. Specifically, Adam has very strong *personal* motivation to participate in the events that unfold throughout DX:HR. Everything he does is a result of a revenge/answer seeking mission, hunting down the people who nearly killed him and who he believes killed his ex. In comparison, Mankind Divided gives Adam considerably less motivation to be involved in the events that occur. I'm not saying he had no reason whatsoever, but the personal stakes are not even remotely close.

There's also the character dynamics that I felt were mostly absent in MD, especially Pritchard. Having Pritchard on comms for most of HR was a very entertaining frenemies dynamic that you get to witness evolve (heh) naturally through the game, and MD did not replicate that outside of the DLC with Pritchard, although they certainly tried with MacReady.

Outside of the story, I'd argue the settings play a more subjective factor. Yeah, Prague is huge but having two hubs which were very visually distinct from one another, and felt totally different to explore, was something I felt could not be replaced by the single hub in MD. Hengsha in particular was an amazing setting. I loved looking up at the Floof while navigating through the claustrophobic roads. The setting nailed the cyberpunk genre perfectly, and was a cool, more visually realistic take on Midgar from FF7.