Easy one for me... Peter Stokkebye’s English Oriental Supreme. It gets generally good reviews (so I guess it's not really "underrated") and sells pretty well, but it's not as popular as their Luxury Navy Flake, Luxury Bullseye Flake, English Luxury Flake, and Proper English. Every time I smoke it, I wonder why I don't smoke it more often.
2 that others have mentioned:
Arango Balkan Supreme, one of my favorites and so forgiving to smoke.
Balkan Sasieni, fantastic price on this normally so worth a try if you haven't.
Of course, Amphora, Sail Yellow ,St Bruno,Mellow(Special)Virginia-Uk,Mc Barens Golden Extra,Comune of Italy, briefly most anything that comes in pouches.
Brown Bogie has the fullest, spiciest VA flavor of anything I've smoked, and the nicotine is beautifully overwhelming. I wish I could smoke more than a small bowl, but even it kicks my *ss!
I second Brown Bogie. It wasn't underrated in the UK at one time as it was smoked and chewed by countless working class people in the early-mid 20th C. Lovely smoke though. I'd also say the UK OTC flakes are still worth smoking and very underrated. Generally the flake versions of St. Bruno and Condor and so on are much better than their RR counterparts.
My vote however goes to Ogden's Walnut Flake, now Ogden's Original Flake. A really classic British VaBur. If I had to pick a second I'd say Murray's Warrior Plug
Samuel Gawith Kendal Plug rarely gets mentioned. It is basically their Best Brown Flake but pressed into a plug. I like it because I enjoy playing with plugs.
I am going to second paulie66scandina in saying that a lot of pouch blends are underrated.
John Bull and Peterson Wild Atlantic are both quite good(and actually pretty similar).
Outside of pouches, Murray's 1921 Cunningham is a really agreeable Va/Per that I almost never see mentioned. Also Comoy's Cask #1 and #11 are really smokable English blends.
I am not sure if these are necessarily underrated, but infrequently mentioned - Commonwealth and Bothy Flake are two of my favorites from Samuel Gawith.