My vote goes for the ribeye every single time.
For me, the absolute best tasting, best textured (thanks to the amazing marbling) part of the entire cow is . . . the spinalis dorsi - period. The spinalis dorsi is the little strip of beef on the outside of a cut of ribeye that has the thin strip of fat that separates it from the rest of the ribeye. This part of the cow is the be-all, end-all.
Here is a picture of a ribeye. The spinalis dorsi is marked with the number 2.
Fat is your friend. Fat is part of what makes beef taste good. A good ribeye shouldn't have any huge chunks of fat, but rather marbling - which will melt and become indiscernible. The only large amount of fat that should be in your ribeye is the strip separating the spinalis dorsi from the rest of the ribeye (the longissimus dorsi, multifidus dorsi, and the longissimus costarum). Get excited, though, when you find this strip of fat because this helps you locate the coveted spinalis dorsi, which is just on the other side of it. So, marbling is nothing to be worried about and the strip of fat you hopefully find in your ribeye is your roadmap to the very best that beef has to offer.
Tell your mom that, since she's the best, only the best would do - which is why you chose the ribeye with it's amazing spinalis dorsi.
Now, if you really wanted to give your mom the absolute ultimate steak in the entire world, you could buy a whole ribeye (USDA Prime, Kobe, or even Mishima if you can get it), cut off 5 to 7 spinalis dorsis, and glue them all together with transglutaminase. Now THAT would be the ultimate steak.