I guess, for children around the age of 5 or so, the best way to introduce them to The Muppets is through Sesame Street, obviously. But if your child is already familiar with Sesame Street, you can show them the "old school" seasons as well. It ought to bring up a fun discussion about the differences of Sesame Street then and now.
With that said, I'd suggest that you
don't show them the earlier seasons - the first three, that is. Sure, that's when we had all the more memorable stuff that the majority of us fans have had implanted in the back of our brains like the Number Song Series, Rubber Duckie, C is for Cookie, Mad Painter, Alligator King, classic E/B sketches like Banana in the ear, broken cookie jar, etc. Yet there were a lot of other things that haven't been fleshed out yet, such as pacing, cast consistency, tone, format, appearance of certain characters, and certain major characters who didn't make it into the show yet. All of which would probably confuse kids today (and even those who grew up with the show after that point). So the best option is season 6, because that's when everything came together.
But I think the main reason I say this is...well, I think I'll let this picture explain it for you:
And this is how your kid is probably going to react (just replace the "Baby Arnold" with the above image):