The most challenging maps for you.

In general, any map that hasn't been well thought out by the author is difficult for me. For example, when the map is almost empty. It's hard to play when you find only one chest and a couple of resources in a week. :)
If you mean a specific map, I can give you this one as an example: "War of Demons is back... AGAIN". If you haven't played it, try playing it as Fortress! It's very long, interesting, full of resources, but... ahem...
:smile04:
For true fans of Heroes 3.