This is a completely arbitrary comparison that I nonetheless found interesting. In being a part of both Heroes and Elder Scrolls communities over the past few years, I've noticed that fan response for both series seems to follow an intriguingly similar formula. In both series:

The first game is hardly played, and when it is, is mostly for interest's sake

The second game isn't the most popular, but still has a die-hard fanbase who to this day play it religiously and champion it as the best in the series

The third game seems to be by-and-large considered the best by a lot of the fans, and is the standard that the rest of the series is held to

The fourth game is a bit of a dark spot. A number of changes were made that proved controversial, and the quality was felt to have taken a nosedive from the third instalment. While it still has its core fans, it is largely disparaged by the rest of the community.

The fifth game is a return to form, with mixed reception. Fans found it better than the fourth game, and still enjoyable to play, but the highly changed gameworld and a few other issues keep it from taking the place of the hallowed third game.

Of course, where it falls apart is that Heroes IV was considered to be the death knell of the series, whereas Oblivion became wildly popular. Also there is no TES VI. Unless you want to count Fallout 3 ;)

Just an observation, but an interesting one, I thought.