I haven't read that much short fiction on the BSFA longlist yet, but here are a few of my possibles:
Tim Maughan, "Special Economic Zone" (Medium)
Sam Kriss, "Manifesto of the Committee to Abolish Outer Space" (The New Inquiry)
Alyssa Wong, "Hungry Daughters of Starving Mothers" (Nightmare)
Yoon Ha Lee, "Variations on Apple" (Tor.com)
Alix E. Harrow, "The Animal Women" (Strange Horizons)
May add to, or even subtract from this list.
Tim Maughan's probably isn't science fiction and it's definitely not a story. It's probably the most solid contender for me.
Yoon Ha Lee is one of the few writers whose language I feel actually deserves, at least intermittently, those common encomiums "rich" and perhaps "poetic" and perhaps even "lyrical." (Especially the more disintegratingly courtly and/or Metaphysical kinds of lyricism: Petrarch, Donne. Often prose described in these terms strikes me as a sleazy and linguistically predictable attempt to woo by introducing heightened straightforward sensuality -- and introducing it at every opportunity and in multiple dimensions (diction, prosody, subject matter). I am depraved and easily wooed and so it sort of works, but the hook-up is regrettable. Whereas Yoon Ha Lee's cryptic similes can actually assail the readerly nostril with the sickly-sweet reek of foundered courtships).
Sam Kriss's story has a brief appearance in my economic speculative fiction listicle.
BSFA's new nomination system
How to reform the Hugos
The best looks back at 2015 of 2015