Boogie Nights was probably the most common entry point in the past but I reckon that the latest generation of movie enthusiasts have learned about his work through There Will Be Blood. While it's one of his most ambitious works, most people seem to appreciate its ambition and the overall virtuosity. And it isn't particularly difficult in comparison to his last 2 films. Hopefully my perception isn't altered by the little PTA-bubble that I inhabit when I say this, but I think it's probably his most well known work right now.
I used TWBB as an introduction for a number of friends and family members. That said, Boogie Nights is always going to be great starting point provided they aren't bothered by the whole porn thing- and I don't think they would mind the influences thing really.