Throwing the question "What is the best cave?" into a forum of cavers is similar to asking "What is the best mountain?" to a bunch of Mountaineers, "What is the best view?" to ramblers etc.. Unless everyone you're asking has done the same trips then you're going to get a wide variety of answers drawing on the personal experiences of the respondants - the problem you've then got is to work out the degree of technical proficiency you will require to reach until you are in a position to have a go at the most frequently occurring "best cave(s)".
There's no harm in asking such a question but I think it's fair to say that you won't get a definitive answer to your question - there's lots to choose from if you pick superlatives such as "Largest single chamber", "Deepest", "Highest Entrance", "Greatest length of passageways surveyed", "Greatest estimated volume", "Most visited by cavers", "Great profusions of formations" etc. etc..
On Mendip, the best cave is probably St. Cuthbert's Swallet for reasons already mentioned - it's got a great variety of passages, obstacles, technical elements, formations, water features etc.; it's also hard work! - the Mendip caves with the largest numbers of visitors will be Goatchurch Cavern and Swildon's Hole (in that order) and the cave with a popular "Wow!" factor is GB Cavern.