Personally, I think
Factotum and
Pulp are his worst books. I enjoyed
Post Office quite a bit, but.
Try
Ham on Rye for the story of his childhood - lots of good humanity in that one, both humor and pathos.
Hollywood is hysterical every time I read it - it's the story of how the movie
Barfly got made. Those are my two favorite novels of his.
The short story collection
Tales of Ordinary Madness is pretty awesome, and how I was introduced to his work.
Burning In Water, Drowning in Flame and
Last Night of the Earth Poems are my favorite poetry collections.
The guy was prolific as hell and not concerned with each individual thing he wrote being a world-ended in quality. The result is, yes, an often mixed report with a fair amount of mediocre material. But digging is half the fun, and I've never regretted moving through as much of his bibliography as I have.