I love Fry and Laurie. It's my favourite sketch show of all time, which is some kind of achievement considering how many I've watched. A perfect exaple of smart, well written comedy, that doesn't have to rely on being too surreal, or having to fill the show with comic characters.
I don't agree that Series 3 wasn't good, I thought it was just as good as the first two, if not the best series of the lot. Series 4 wasn't as good, but it still had some excellent sketches (the newsreader coughing fit is one of my all time favourite things they did).
Hugh's songs were also a fantastic addition to the show. I've found it really difficult to decide what my favourite ones are, but I would say the America song from Series 1, Where Is The Lid from Series 2 (and the obituary after it was genius as well) and The Nice Rap from Series 4.
Here's links to some of my favourite sketches -
http://www.youtube.com/watch?v=lyHSjv9gxlE - America song from Series 1
http://www.youtube.com/watch?v=Cyk_d_1VSas - Where Is The Lid song from Series 2
http://www.youtube.com/watch?v=CJW_yTbYGoI - Michael Jackson sketch from Series 2
http://www.youtube.com/watch?v=eEkjiF_UilE - Countdown Parody from Series 3
http://www.youtube.com/watch?v=9bUOpjamRB0 - Question of Sport parody from Series 3
http://www.youtube.com/watch?v=7C8GWIV4jzA - Bookshop sketch from Series 3
http://www.youtube.com/watch?v=QHKkuIU0OJc - Coughing Fit from Series 4
http://www.youtube.com/watch?v=XdRfhARwGoI - Be Nice rap from Series 4
http://www.youtube.com/watch?v=26FKgGK84po - Don't Be Dirty sketch from Series 4
As I mentioned in the Monty Python thread, there's so many good sketches that it's so difficult to pick out the best ones, because the quality of the show is so high. It's always a sign of an excellent show when you can think of 20-30 other top quality sketches off the top of your head, yet you can't remember the bad ones.