In order, my faves are:
1. Monty Python
2. The Marx Brothers
3. Reeves and Mortimer
4. Laurel and Hardy
5. Beyond the Fringe
--------------------------
6. Fast Show
7. Big Train
8. A Bit of Fry and Laurie
9. Not The 9 O'clock News
10. Q
Top 5 are 'lock on' in my opinion.
For me it's The Fast Show, head and shoulders above any other sketch shows I've ever seen.
Big Train
Not Only But Also
Fast Show
Kids In The Hall
League Of Gentlemen
A Bit Of Fry And Laurie
Any Reeves And Mortimer
Big Train actually looks better and better with age. At the time I remember feeling disappointed with it, but when I watch it now it feels fresh and vibrant and leagues ahead of recent sketch show offerings.
The best bits of Python, Absolutely and Not.
The best Python stuff for me, sketch wise, are the records. eg. Matching Tie and Handkerchief.
Unfortunately too much TV sketch stuff dates badly. eg. Python. Some of it's so f**king dull now.
I also think it's worth mentioning just how good French and Saunders sketch show was at its peak. They've pissed away their reputations in recent years, but the stuff their nineties sketch stuff was very good.
I also used to like Alas Smith and Jones.
Big Train had some absolutely brilliant sketches.
Alas Smith and Jones - I always liked the home video one.
I enjoy sketch shows and even try to write them! But when I try to think of classic shows, I find it hard. Easier if we're naming individual sketches.
But here's my try:
Smack the Pony
Big Train
The Fast Show
Reeves & Mortimer
League of Gentleman
Monty Python
Not The 9 O'clock News
Oh, wasn't that hard 
Oh, and The Day Today, which is a sort of sketch show.
And Jam!
This week I'll mostly be suggesting The Fast Show.
The family running with the cases was just genius!
I'll add The Day Today, if that counts. Which I think it can, just!
The Two Ronnies. How could I have forgotten this? Brilliant. Although by the time they'd finished they'd pretty much run out of variations of dinner party sketches.
On the above note, one of the best NOT sketches that Smith and Jones did, was the Two Ninnies piss take in the last series, with the song by Pete Brewis.
Good shout on Big Train. It was quality. The Two Ronnies as well.
Quote: Leevil @ February 25 2009, 2:04 PM GMTI'll add The Day Today, if that counts. Which I think it can, just!
Absolutely. Love that show. "He's got a foot like a traction engine" 
Quote: Matthew Stott @ February 25 2009, 1:47 PM GMTAny Reeves And Mortimer
What's your fave R&M show mate? I think 'The Smell Of...' was their finest, but I love 'Shooting Stars' and 'Big Night Out' too. It's all great.
Very good suggestions here. I'd go for:
The Fast Show
Big Train
The Two Ronnies
Harry Enfield and Chums
Alas Smith and Jones
Not The 9 O'Clock News
A Bit of Fry and Laurie
That Mitchell and Webb Look
I put Mitchell and Webb on the end, but to be honest it probably wouldn't make my list, there are some good sketches but I find a lot of it quite unfunny. The other ones are great though.