Over the years this answer has changed a lot. I can say that at one point or another every muscle group was my favorite to train.
I think now I'm more inclined to favor whichever muscle is growing the fastest. Whatever pops out the most in the mirror when I go back to the locker room after training. I look forward to getting that instant feedback and seeing the look of the muscle change.
Currently that is quads. For my whole life, I believed that squats had to be the foundation of leg day. Though I don't consider myself an expert squatter to any degree, I wouldn't marginalize my experience in the rack. I can walk in and do 4 sets of 10 with 405 which has been a goal of mine since I was 13. I can do 3 sets of 20 with 315. I've walked 585 out of the rack and did a shitty single with it when I was younger. It's not something I care to attempt again. I've done high bar, low bar, wide stance, narrow stance, front squats, used the Hatfield bar, parallel, below parallel.
But once I was finally able to accept that squats weren't giving me the quad growth I was looking for, I have been making great gains by ditching them and focusing on the principles of growth I know to be true. I was reluctant to ditch them because I thought it would mean I was a pussy or that I wasn't hardcore enough. Once I set aside my ego, I have noticed that I can achieve optimal foot placement and thus optimal load placement by doing front squats in the Smith machine. It allows me to control my tempo better and to work in the eccentric portion of the movement safely and more effectively. I take one set to failure with that exercise then one set to failure on leg press which usually ends up being 4 drop sets with a total time under tension of close to 2 minutes. I used to think if your TUT was in excess of 1 minute you couldn't possibly be training heavy enough or intensely enough. Another thing my young mind was certain about that my aging body has disproved. My quads blow up from all of it and I have begun to notice a density to them that I've never seen before. The muscle looks thicker. It looks ... freaky. And I enjoy it.
So despite it being the most difficult session of the split, I am looking forward to seeing what it looks like when I'm done each week. Actually I'll be hitting them today.