LAMB!

After that I like:
Goat
Pork
Beef
Honestly, its usually a function of the fat content. Lamb is so delicious but its really unhealthy on a regular basis. And of course the cut can matter a lot. Beef can vary immensely based on cut alone.
Fuck! I forgot goat.
Usually, but not always or entirely. Many pork products of course tend to have a higher fat content, but the fat itself tastes different. I think pork fat is tastier than beef fat. I'd rather have baked beans with bacon and sausage than fatty ground chuck and suet.
And venison's quite delicious even though it has a very low fat content-- this often means you have to add fat to dishes made from venison, but still, the taste of the meat itself is unique. And of course filet mignon is not a very fatty cut of beef, but the texture and purity of taste are to die for. If you think it doesn't have quite enough fat for your taste (say you're a Ribeye or Delmonico kinda guy), a nice Diane sauce or red wine demi glace/gravy/au jus will fix that right up. Turkey is lower fat than chicken, but not when you cook it with garlic sage butter rubbed under and on top of the skin and in the cavity.
But yeah-- fat is fuckin awesome. Olive oil, bacon, lard, butter, cheese...when it comes to fat, it's all good, man.