The second half of the 20th Century was far less rich in great humour strips than the first half. Having said that, there were a couple that rank with the best ever.
The only place to start is with what was by far the dominant humour strip of that era, Peanuts. Charles Schulz throughly earned his place in the hearts of millions around the world, with one of the great casts of characters and some wonderfully subtle comedy writing. Some great humour writers would take pride in a strip being taken as against both sides of an argument; Schulz felt that way about one strip that was taken as in favour by both sides, the issue being prayer in school – I guess this is the difference between a satirist and someone with as much human warmth in his work as Schulz. Perhaps his artistic limitations would have been more exposed in earlier decades, when comic strips were a lot bigger, but he found a style that worked very well for him. Peanuts was a magnificent strip, particularly so soon after he’d found his stride, in the ’60s especially. In Charlie, Snoopy, Linus, Lucy and Peppermint Patty in particular he created some of the best known and most loved comic characters ever.
Even better, for me, is a strip with some similarities, Calvin & Hobbes: again, a small boy and an animal, and again lots of regular riffs. The two leads are great characters, as is Calvin’s dad (was it Schulz who thought he’d take over the strip?), and Bill Watterson was a far better artist, genuinely brilliant a lot of the time, both in spectacular colour Sunday fantasy sequences and in precise expressions, especially for Hobbes’ appalled and ironic moments. He was also tremendous with words – a real feel for language’s possibilities. Contrary to Schulz, he would never license merchandising. I suspect an unwillingness to fix Hobbes in one form, to make a decision between his being alive and it all being Calvin’s fantasy world, was behind this: there are strips supporting and denying both interpretations. It doesn’t really matter – it was a glorious strip, and he stopped before any significant decline in quality, so its run is perhaps the most perfect ever.
Cheating a bit, but I also want to mention Gary Larson. He rarely ventured into the strip form, generally offering a panel gag, but the Far Side cartoons are among the funniest ever produced. He made dazzling use of animals of every kind, but seemed to be able to create hilarity from nearly any territory.
And if I am mentioning single-panel series cartoonists, let’s fit Giles in here too. His political points were sometimes rather tedious, but the raw chaos of some of his best panels, most often those centring on the large family he depicted so beautifully, is irresistible, and the mighty grandmother is an unforgettable creation.
Collections: you still see those lovely old Peanuts paperbacks around some, and now there are prestige collections appearing of all of it, in order. The Calvin & Hobbes collections should be easy enough to find too. Far Side collections are easily available. Those lovely Giles books are sometimes found in charity and secondhand shops, but they have become more collectible and therefore expensive, and the late editions are much less appealing.