Other than some of the already mentioned (Tawny Roberts, Jessica Drake, Kendra, ...):
Marie Luv is always pretty ripped, if you may say so.
Amia Moretti seems to be in really good physical shape, though she's a really petite girl.
Jesse Jane, although always on the verge of being too skinny, is pretty "lean and mean" too.
Kylee Strutt definitely works out a lot and isn't too muscular yet.
Celia Blanco and Logan James had really fit bodies. So did McKenzie Lee and Emanuelle Diniz in their prime.
Even Jayna Oso had a really nice body in her "best years" from mid 2003 till the beginning of 2005, till she kinda let it slide.
Mya Diamond, Andy Brown and Caroline De Jaie were definitely some of the hottest and fittest bodies on the European meat market.
And amongst the "new girls" there are Halia Hill, Veronica Clinton and Kenzie Marie.
Not to forget two of the probably best Japanese pornstars: Noa (Torigoe) and Riko Tachibana (names are in American English order).