Smash, I haven't played the HOMM series. I completely missed the first 4 and then when 5 came out I played the demo and didn't like it. I thought it was slow and cumbersome. So I am yet to play HOMM V.
Disciples II on the other hand is one of my favourite games. I played it and all the expansions (except the Elven one) many times. I see KB as closest to Disciples II in gameplay, with KB being superior. Even though D2 has more of a strategic overview (with the resources, castles etc) there were a lot of things I found annoying in time, such as the way you could only take one hero to the next map. The turn based play also got very tedious at times (I mean on the strategic level, not the tactical battles). And the battles were more limited than they are in KB.
So it might be true that one of the reasons I love KB so much is that I haven't played the HOMM series. But for whatever reason I have played this game now well over 200 hours now, and that's more than more than all but a handful of games in the last ten years.
