Mott The Hoople: Mott (1973)
Forced to pick just one, I'd probably go with 1973's Mott, largely based on the inclusion of two of their finest songs -- though, really, pretty much anything from 1971's Brain Capers through 1974's swan song The Hoople is on equally solid (or shaky?) footing. The first few albums, a blend of British folk rock and hard rock, have their moments -- I think their self-titled 1969 debut, almost Dylanesque in parts, is particularly engaging, though uncharacteristic of their later sound -- but it's on Brain Capers that their more glam-oriented sound really gelled. And then, of course, David Bowie handed them one of the greatest pieces of songwriting from his long career, and 1972's All The Young Dudes took them to a whole new level. And while Dudes is pretty great largely based on the title track and a respectable cover of Reed's "Sweet Jane," the originals aren't exactly riveting.
In contrast, Mott saw the band's songwriting rise to meet Bowie's challenge. The opening track, "All The Way From Memphis," is just about the best thing they ever did, a rollicking anthem with one of the greatest sing-along choruses of the era, something that always deserved far more classic rock attention than it received. (Personally, I find the one limitation of the song to be the extended sax contribution from Roxy Music's Andy Mackay, which starts out pretty great but veers into more grating territory -- I prefer an earlier fade to the song myself.) Every bit as great is "Honaloochie Boogie," perhaps their purest piece of glam songcraft, the spacey vocals effects on the verse leaping into a tremendously catchy chorus, with varied guitar sounds and, once again, Mackay's sax (used to great effect here).
A bit of a drop off beyond those two, but "Drivin' Sister" is a fun T.Rex-y romp, one of their better upbeat pop-rockers; and there are some lovely ballads (most notably the autobiographical, epic, Bowie-esque "Ballad of Mott the Hoople"). The album is still better suited to a singles mix, but it's never less than listenable, and often glorious.
Here's "All The Way From Memphis" (studio version with poorly-syced live footage):