I think this is really hard. Personally I don't think there really are best/weaker series per se. Every series has got its brilliant episodes and ones that you perhaps could consider as 'lesser episodes'. I do agree with you that series 4 is brilliant, and definitely a stronger series as a whole, although I have heard people saying that they didn't really care much for 4 and absolutely loved 5.
The only episode in series 4 that doesn't work really well is 'Paris' (something Jen said herself as well in the audio commentary)
It's still a fantastic episode and it has some extremely funny scenes, but it is true that it doesn't work as good as it could be when you separate the 2 characters.
When you look at series 1, it's clear that the pilot, 'Fashion', is a bit separate from the rest of the series. All the actors are still trying out their characters, but nontheless it's a great episode, and definitely a good pilot. It definitely leaves you wanting more. But you can see, if you compare series 1 to later series, that the characters definitley evolve a lot throughout the years. Patsy's habits definitely get worse, more drinking, more smoking. Edina gets a lot more obsessed with fashion and celebrities, and Bubble just gets more and more weird compared to the first episode, in which she's still quite "normal".
That's what I like about long running series, with not too many episodes (a concept the Americans don't understand at all). The actors change, they get older, the world changes a lot too, and thus the series changes too because of that. This prevents it from ever getting boring. Not that anything written by Jennifer Saunders is likely to get boring though.
I could go on for hours

Perhaps we should start separate threads for each series, so we can obsessively over-analyse them
