Porcupine Tree - Trains (track) or Lightbulb Sun (album)

Lots of people would say In Absentia as a gateway album as it’s their most popular. I think Lightbulb Sun is a little more accessible as it isn’t as full of heavy riffs and extremely dark lyrics (Russia on Ice excepted). Personally Deadwing is my favorite, but it has some of the same barriers to accessibility that IA does, plus a ten-minute opening track rather than a catchy, right rock song like Blackest Eyes.

Lazarus is also a great choice for a gateway song; that one is generally a crowd pleaser for those not into overly heavy music.

Also, Nick Cave and the Bad Seeds - Dig, Lazarus, Dig!!!