it's not right to say that HoMMIV was worst. it's a pretty adequate game on it's own, BUT it's doesn't continue the tradition of other hero's games, if you'll excuse me saying so- it's more like MoM than HOMM
and HOMMIII was just the latest one before this- so, sinc it didn't have any *risky* features, and had most the gamers complaints (at the time ) addressed it was naturaly proclaimed best (like II in it's time).