My personal favourite was either Series 1 or Series 4. Series 1 because it had the best actual business people at that point, not just quirky people - and there were still lots of funny moments, it was also the one where you couldn't work out who was going to get fired or win that easily as you can now. Series 4 because it was the funniest series and a really good candidate won!
Series 2 was my favourite, Syed and Paul were just awesome to watch together, as well as the business aspect of it still working. Series 4 was probably the last series that any of the characters actually stuck with me, since then the rest just blend into one bland mess. I couldn't even tell you the name of one of the people on the current series right now.
Series 2 was very good - I think afterwards the series's became a bit more and more centred on controversy and entertainment, which defeats the objective.
Series 4 (I didn't watch 1-3 mind) was immense. It had Raef and Michael Sophocles, the Marrakesh task, Margaret tarnishing Edinburgh uni forever...amazing. So far, the new series is my least favourite. Everyone's pretty bland