I like the K-22's, and the post-1957 M17's.
I'm not as much into the underlugged barrel M17's and M617's; the bore is so tiny that it already has plenty of muzzle stability, and with the underlug it is just too muzzle heavy.
Well, the 4" full-lug isn't bad, but the 6" and 8 3/8" full lugged .22's are too much.
I really like my M18! Sweet shooter, too bad it's not a round butt.
Oh, and all the above are adjustable sighted K-frames.
If you're ambitious, and willing to wait, you could hunt for a M45, or pre-M45, the .22LR M&P with fixed sights. Like a skinny barreled 4" M10, only in .22LR!
I know you stated you want a long barrel (and I think medium frame was implied), but the J-frame M34 (blue) and M63 (stainless) are great fun in 2 and 4 inch.
-Kframe