That's a difficult one to answer. I really enjoy loading up rifle rounds because of so many different variables and things to play around with. If you mean enjoy because of ease, I'd have to go with 38 spl and 357 Mag.
Kinda depends on how I feel at the time, but I've reloaded a train load of .44 Magnums. I have 5, .44 Magnums that I like to feed a variety of ammo, and cast bullets for them too. But, lately I've been playing with my .223 and loads of WC820 and also trying cast bullets in it.
Sometimes I think I've been neglecting one of my guns (or calibers) and do some shooting/reloading (45 ACP and .357 Magnum are also near the top of my list), but always come back to my .44s...
Lots of calibers but it goes back to what I started with, 45 for pistol and .308 for competition and deer hunting. I guess you could say they are my meat and potatoes and everything else is side dishes.
I particularly like loading my precision rifle ammo. Particularly my .308. Just something about the attention of each and every teeny tiny detail and making certain each and every charge, bullet, case, and OAL is perfect. and then hitting the range and firing 10 consecutive 5 shot groups that measure between .6 and .8 at 200 yds. I love it.
I just enjoy reloading in general. I find it rather relaxing, and the time passes by so quickly. When I get the first of my 2 AR's built, I'm pretty sure loading 300 blackout will be a favorite for a while. I have been buying brass for it every chance I get. My only choice around here is to either mail order it and pay hazmat fees, drive the 90 minutes to Cabelas and prey they have it, or mail order the brass with no hazmat fee and buy the rest of the stuff local. I have dies, brass, powder, primers, just waiting until the guy at my local shop goes to pick up his order with my order for a good supply of bullets.
I enjoy loading the .327's; they seem to be the easiest to get a good flow and few problems. Something I enjoy about loading 38's with 110gn Speer's too though. I have a few loads that I get a little more excited about loading. None that I dread though, they're all enjoyable.
I like loading 300 RUM. There is something about pouring 106 grains of powder into a case, capping it off with a bullet, and then feeling the weight of the finished cartridge in your hand; all the time knowing how much power is bottled up inside it.