I have been wearing this thing so often, it deserved its own little ode.
The HELMUT Helmut Lang Asymmetric Bra (in black). It's my bandeau bra of choice. Offers decent support, doesn't slide all over the place and has that special detail that makes it stand out above the rest.
At $90 it is steeply priced for what it is - a modal jersey bandeau bra - but its versatility makes it indispensable. It just looks so damn cool underneath any tank/loose top/casual dress. Hell, I even wore it hiking!
I bought mine from Shopbop but it hasn't been available in any size except XS for weeks (for reference, I bought the size S and I wear a size 32C bra. My mom, 34D, bought size L in white and it's perfect).
It is currently available in all sizes at Saks (link), the Net-a-porter UK site (link), or you can indulge in the gorgeous tank top version (here).
Is it expensive? Yes. But it's 100% worth it for such a cool, unique piece. I want it in white.