For me it’s Aguila Pistol Match. It leaves the breech of my Beretta 87 Target cleaner than any other ammo that I’ve tried. I think that’s mainly because its MV is only 925 f/s, at the low end of standard velocity, so chamber pressure is low. Its Eley primer might possibly be part of the reason too (Aguila is owned by Eley).

I like this ammo for other reasons too. The plain lead (no coating) bullets are perfectly formed, and firmly clenched in the casings. Its accuracy is at least as good as any other moderate price ammo that I’ve tried.

It is nice but damn stuff is sky high the last I bought ten years back.
I am 22 poor ,still have a couple cases I got from The friendly folk in anniston alabama years ago.
C M P used to have deals.

In the late '60s or early '70s, there was an article in Handloader Magazine by a fellow that was a research engineer for one of the ammo companies that manufactured .22LR ammo. Their claim to fame was producing the most accurate .22 ammo here in the US. They did it by finding a powder that resulted in essentially 100% combustion. About as clean as you can get.