Thinking about this has made me realise how important personal/group memories are in how fond you are of a pub. Probably the one I have fondest memories of is objectively one of the most average bars in the festival, the Hop and Liquor. It wears its theme so lightly that it's possible it doesn't even have one and I've made it up, but I think it's meant to be like an Old West saloon. They serve the bog standard drinks - your Carlsbergs etc, nothing fancy. They play inoffensive crowd pleasing music. And yet. For some reason, probably thanks to its location near the Pyramid and nearish where we camp, over the years my group has always spent a decent amount of time there waiting between acts, quenching our thirst, or just plotting what to do next (never waste time planning without a drink in your hand). And if you spend a decent amount of time anywhere at Glastonbury funny and fun memories will start to accumulate. So for that reason it's somewhere we all look forward to revisiting every festival, to sit in the little 'beer garden', talk to some strangers and watch people go by. But if you were to ask me which is the best bar at Glastonbury, it probably wouldn't even make the top 10.
Speaking more objectively, the Avalon Inn is a thing of beauty, and that whole area is a delight.