Favorite hero from Dungeon.

I wouldn't say that; only she can effectively eliminate the engineer (ice block + deadly cold), plus she makes an excellent secondary hero.

You're confusing her with Kifrah;)
And Irbet is indeed weak. If there are resource nodes on the map, her specialization doesn't look convincing.

Look at the first pages, Mdan's posts; this guy can defeat anyone with Lethos.

Unfortunately, this is not the case :)

A lot has already been said about Lethos; he is the strongest hero in the Warlock faction, if you know how to play him. But the Sanitarian is only effective in the early stages of the game, and on small maps.

I agree with that. The speed of Lethos's development has certainly decreased in 3.1 compared to 3.0, but he still remains the fastest Warlock. And in particular, he is a practically indispensable asset against Orcs. His Plague and Cursed Land spells are very effective at disrupting their rage.