I believe the leanest cut of cow is eye round. Look at it in the package next to a sirloin and you'll see there's hardly any fan in the meat, just a thin rim of it around the outside. I like eye round steaks grilled, but I have to have them "medium" to "medium well" as opposed to medium rare, which is how I like just about every other cut of beef. And eye round takes longer to cook to the same level as sirloin, I think because there's hardly any fat in the meat. It seems more dense than other cuts. It ain't cheap, either. My wife does NOT like eye round steaks grilled, she prefers to pan sear them. I don't like them like that at all, but whatever works!