For me, it's Procession. Usually overlooked as a B-Side, (despite the 7" in question only labelling the two sides as "Hard" and "Soft"), I love it. It's not my favourite NO track, but the ones I like better I don't think are underrated, eg Elegia, Ceremony, side 2 of Technique, Leave Me Alone, etc because most fans rate them quite highly.
Procession is a great tune, from the opening/closing synth drone to the excellent lyrics. Here is Barney stepping out from the shadow of Joy Division: "A mass of harmless attitudes/attack them or subside/no matter what you say or do your heart meets you/late at night". Easily as good as anything he's written since and much better than a lot (I mean, compare this with, say "state of the nation/our salvation" or "the sun was beating down on me ... I had to go to sea", etc and you could hardly believe it's the same writer!).
Although EGG is great, I think the fact that Procession was dropped for the 12" is such a shame. And Substance didn't help by relegating it to the B-Sides section.
All in all, I reckon that constitutes "underrated".:)