Well, where to start with this game ...?
Diablo is a classic that took roguelike games to the common man, with nicer graphics and entertaining sessions where you can choose to play for half an hour or four hours at a stretch.
Diablo 2 was a huge success that was better at everything.
Diablo 3 was a big disappointment to many at launch. It requires an Internet connection, even in single player mode. It was designed now by people from the WoW world and had many influences from there. But over time, after the controversy and drama and kicked a game designer behind, Diablo 3 has evolved into something quite different. Perhaps the best in the series. Or, yes, now it can at least be discussed, something it could not do before. But you must have the Reaper of Souls. Without expansion, the game lacks almost completely replayability after you have reached level 60th
Forced to rate WITHOUT expansions in mind. In that case, Diablo 3 the game for too many patches needed to launch quality a pretty fun and entertaining game, but I would not consider it better than Diablo 2 only on its own feet.