The _easiest_ that I have found (and believe me I was a dummy when I started) is Metasequoia. It's free.
I use v2.24 to model (nice tools) and save as .mqo.
Then I load the .mqo into v2.1 which can export as .X.
Both can be found at
http://www21.ocn.ne.jp/~mizno/main_e.htmlI also use Milkshape (needs some patience to learn) to create animated characters, and it exports as .B3D.