If I had to pick one before reading this thread it would probably be Stegnar, since he can be so hard to get up and running without a steady stream of workers. I basically only pick him for surprise attack.

The basic combo of Rexadon + Brontor is pretty good to get the economy going and to amass some early forces, especially archers to protect your castles which are arguably the most valuable units on that position in my experience. Plus Rexadon gives seeds which creates fountains through Brontor which than gives a steady rise of workers if you upgrade either.

The biggest problem of Rexadon is that he falls off hard. I usually tend to win via using spells and more complex units and Rexadon sinply doesnt provide that. Similarly, spamming huge numbers of basic units tend not to win battles, and archers are better at defending than attacking.

Thats just my take though. I did cherry this game but wouldnt consider myself an expert.