My favourite is an old MK 11 bought off here. It was re blued by Colin Malloy and fitted with V Mach kit (tuned by them) I later fitted a short threaded 20 cal barrel (originally 22), tweaked spring set (I think I bought a slightly longer spring off V Mach, can't remember now)
It has an Edgar sirocco moderator, which gives a great cocking grip and is really quiet, and a Nikon EFR 3-9 x scope in two piece sports match mounts.
I like this one as it's quite compact and a bit lighter than usual, I've often wondered if the re blue lost metal or something, plus the old beated up stock may just be a particularly light one.
It's my all time 'go to' never loose zero, reliable , doesn't power creep 'handy rifle'
I took one of my best shots ever with this hw80, a squirrel on the ground. This was around the time I was discovering 20 cal at uk power is a really cracking choice with FTT weight pellets (I find until past hunting ranges, the trajectory is great, more wallop than 177 but greater PBR than 22, unless using very light 22s, which personally I don't like)
When I first had hw80s I sold them on as big and clompy, unwieldy, slow to shoulder.
Glided ones are a fair bit lighter, and with shorter barrels 12, 13, 14 inches handling much improves.
My last one a full length old 22 has open sights. With that original 19 inch barrel on, it is too heavy for me when scoped up, I just have it as a big ole 22 open sighted rifle, classic fun.
I think that they can be really great and practical rifles, and with a short stiff spring to get that ruddy great piston moving, have a nice cycle.
I prefer them in 177 / 20 at uk level, as that amount of spring seems to be the best result at uk power (I do have an fac one as well, low 20s fully glided in 22, really set up for chest (heart) shooting squirrels, and Crows)
You can never beat an 80 IMO, but recently I have been shooting an old 22 77K with 25mm internals, with great success and pleasure at Iden Ferns, just for fun, which has an uncanny like and accuracy with HW branded FTT (HW Pellets, the H&N variety)
Looking for TO-6 Trigger unit unmessed with or T0-6 kit for 34