Enell - as everyone else said! I'm a 36G (that would be four D's) and I wear their size 2. It is VERY snug, sort of uncomfortable at first, but it will strap the girls down and hold them tight. I run and ride horses and I wouldn't wear anything else. I just bought a new one from See Jane Run, as my old one was wearing out on the hooks (had it over a year and I wear it 4-5 times a week, alternating with a newer one). Totally worth the money. One word of advice - get the right size. Don't size up thinking it'll be too snug - I ordered a size 3 by mistake and it was useless. Sent it back and got my size 2. And even though size 2 says it only fits a DD, it fits me - often larger breasts are very soft and they pretty easily squish into the Enell.
For slightly lower impact activities like the elliptical machine, I love the Moving Comfort Maia bra - also got that from See Jane Run. Bit less uniboob effect (the Enell is a total boob smoosher) and a bit more comfortable. Highly recommend both!