I think this one's the funniest. I know each series has its fans, even the first one, but this was such a change of tack compared to the first series that it really set the formula for series 3 and series 4.
In the first series, Edmund was slimy, weedy, stupid and unpopular. Baldrick was the clever one, who saved him on more than one occasion. The series cost a fortune to make - every episode had outdoor shots on castle ramparts, in monastery grounds, in woods and on hills. Then Ben Elton joined the team, and this series was filmed entirely in either Blackadder's front room, Queenie's chambers or in the Tower of London dungeons. And Edmund went from being supremely stupid to the sly, cunning genius he has been ever since. True, he retained his loathing for everybody that he had in the first series, and he still is only ever looking out for himself, but he is an entirely different person compared to the Duke of Edinburgh. And, because he's now clever, Baldrick has become very very stupid. Percy, of course, is pretty much the same as he was in the first series, just more so. Whether Edmund is getting married to his manservant, executing the Queen's enemies in the Tower jail, travelling around the world with a legless sailor, selling his house to pay off his debts, having a piss-up and a puritan meal simultaneously or trying to escape from a German kidnapper, his exploits are hilarious. All the supporting cast are superb as well. As I mentioned above, Baldrick has become the lovable idiot that he still is, Percy reminds me of an Elizabethan "Gareth from The Office", Queenie is so girlish but with a wonderful mean streak, Melchett is the odious arse-licker to the queen that Edmund aspires to, Nursie is nearly as stupid as Baldrick, but cute with it.
Series 3 and 4 are still very good, but for me this is the best.