I’m really sorry that John Smith’s was the first bitter given as an example here; it’s (IMMHO) not very nice at all. The local brewer is always the one to try in the UK such as BRAINS beers in Cardiff where I grew up. Their bitter is called ‘light’ their mild called ‘dark’ and their best bitter ‘S.A.’ after the brewer… All good. My favourite beer of all is co-incidentally Samual Smith’s - the brother of John, who remained small and independant while his brother went national. Not terribly easy to find, but a great pleasure to do so.
Jennings - the local brewer in Cumbria has some good beers too.
MiM
Good list, TPWombat but you forgot to include Charles Wells’ Bombardier and Shepherd Neame’s excellent brews Spitfire and Bishop’s Finger. And then there’s Fuller’s London Pride…oh, I could go on!
As for the dreaded John Smiths, it’s a kind of last resort for me as far as bitter’s concerned, it rates on a par with Banks’s bitter which is akin to the devil’s bathwater.
Oooh, Spitfire, good addition 
Fullers London Pride???
::rushes to bathroom, vomits, copiously::
Are you unwell, dear? Was it that kebab you had last night?
I’m going to have to ask you favored folk to stop mentioning Bishop’s Finger, if you please. They no longer export the nectar to the colonies, and I’ve been going through withdrawals for several years now. Either stop talking about it or start shipping some my way!
That sounds familiar. In the bar I used to hang out in as a young man in West Texas, I would on occasion down what was called a Prairie Fire, which was a shot of tequila with seven shakes of Tabasco sauce in it. Curiously, it almost always gave me the hiccups, and the bartender would put a couple drops of something on a lime or lemon slice and have me suck on it. Worked every time. I can’t remember what the substance was, but it could have been bitters.
I have been assured by a Canadian friend here that Old Speckled Hen is top quality, but he didn’t refer to it as a bitter. It was on a menu under Beers in The Road House barbeque place here in Bangkok. We were going to order it, but they said they were out. I’ll have to look around for it in the Western-oriented grocery stores; I’d never heard of it or seen it here before.