It can be difficult to choose given that different people will be turned off by different things, but I've found a few different good starting points:
Ignorance of The Law is my Excuse, Episode #64. For me, this has been one of the best general-purpose episodes to start people off with, it covers a wide variety of F-plus themes, it's relatively clean, it features of one Lemon's best voices that debuts just four episodes earlier in Any Cling, and it's just extremely accessible.
The same goes for Today's Progress was Yesterday's Form Field because it too has that variety and also gives us the first Bozarth.
and
The remainder of my Time Will be a Series of Grunts also due to its wide variety and surprisingly varied level of pop-culture reference points.
For people that want things more consistently on-topic, Any Cling is weird enough to fascinate just about anybody, and if you don't mind starting at a cut, the Garfield Fan Fiction one is incredible in the same way, no offense to Van Hozel.