The people who actually read my blabbering on this forum will probably expect me to say 1-4. Most of my tongue lashings are usually saved for the later series, and I've never found much wrong with Series 3.
But I'm really not very sure (and am hoping by the end of this post I make up my mind). In some ways, I'd like to not answer the question or daintily sidestep to my left around it. I wouldn't really put want to put Series 4 in with the first three, since it seemed to mark something of a transition. Also, Series 8 doesn't completely fit with the three before it in style.
There are definitely things I don't like about the later series. A lot of it is too linear, too based on success on certain obstacles, too obvious, too defined with respect to good v. evil, for my liking. I think a lot of the mystery of the early series was gone. I rather like the fact that we didn't know what Mogdred's motives were, whereas with Lord Fear things became a little obvious and predictable (admittedly, especially when you're no longer watching it as a 13 year old). I also thought some of the storylines in the later series were a bit lazy, although I suppose one could say "at least there
were storylines". It became far too easy to get to Level 3, in my humble opinion. Hmmm, this getting rather long already, so I'll keep my other criticisms of the later series short. Eye shield. Reach wand. Ridolfo. Lack of good wall monsters. Brother Strange. Ridolfo. Too many spyglasses. Ridolfo.
Having said all that, the early series weren't perfect, and the later series has much to commend them. The graphics were better, I much preferred the later titles and theme music, and the backing music during some of the tenser moments was fantastic. Most of the puzzles themselves in the later series were gorgeous. A lot of people mention the sentimental value of the early series, but for me most of any sentimentality is centred on the latter ones, since those are the ones I firmly remember watching the first time around. I think the later series had the best complete set of characters too, especially on the Opposition side. I do think that the humour was occasionally pushed too far, but I still enjoy watching Sly and Lissard a great deal.
As I mentioned before, despite having some fantastic elements and setting up the programme in all its glory, the early series had some aspects I didn't like. Some are specific to specific series, and some of my criticisms of the later series hit Series 4 too. Hmm, enable criticism short mode again. Often slow paced. Sometimes more of a story would be nice. Supermad Treguard. Velda. Merlin taking about two minutes to cast a spell. Velda. Oh, throw in Velda and her overacting again for good measure.
I suppose we could go on forever on this type of topic. I haven't even mentioned either of the characters who make up my username here, for example. Although it's a tough decision, I'll go for 5-8 since I like doing what people don't expect.

"Sometimes of the Grey, but always of the Green, dearie. Shurrup Brollachan! Have you seen Festus, my dear? I need to take him back to Cornwall. " [cackle]