For me, free verse is the most challenging form of poetry to write. That may sound strange, but when you think about it, what's the difference between a free verse poem and prose? There is a difference, right? Otherwise, all prose would be poetry and all free verse poems would be prose. Is it simply the way the words are arranged on the page (or screen)?
There are poetic "tools" a poet has in their toolbox, things like alliteration, enjambment, meter, rhyme, metaphor, and many others. What makes free verse free is that the poet is free to use any of the tools in the toolbox in any way s/he likes. That's the challenge, otherwise you just end up with prose broken up into goofy, random lines.
As an example, see "The Masque of the Free Verse" on the Sonnet page under English sonnets.