If we can stretch back to PS2, I think it's probably the Disgaea games. Not because they're my favourites by any means, but because grinding for endless hours is a big part of the games. I haven't touched one in a while, but I'm pretty sure I clocked at least 300 hours on Disgaea 2 alone, which is nothing to say of Disgaeas 1, 2, and 4. Usually I would just throw on a movie on my laptop and grind endlessly while not paying much attention to the game.
Was it entertaining? Well, I did it, so I suppose it must've been...