My Gawd.I loved Ham on Rye.
Well, the novels are certainly the most accessible (in general), but don't shun the poem or the short story. If you're just not into the poem thing, then try the short story. My favorite is South of No North, but they are all good.
If you must stick with the novel (they go fast, and then they're gone, you know, until you read them for the second and third time...), then here's a general hierarchy (in no particular order within groups):
Ham on Rye
Mixed/Not so Good?!!?
I actually love Factotum, some of his funniest lines ever.
Of course this is all very general and subjective. Some think Pulp is his best novel, and I liked that one very much too.