I don't know who the most underrated songwriters could be ! But Bruce Cockburn, Todd Rundgren {especially his Nazz stuff} and Stuart Copeland are underrated. Some of Copeland's songs in the Police are as good as Sting's in my opinion. He actually co wrote some of them with Sting ('Peanuts', 'It's alright for you', 'Landlord', 'Rehumanize yourself') but his own compositions { 'On any other day', 'Contact', 'Does everyone stare', 'Bombs away', 'The other way of stopping', 'Darkness'} were fantastic and provided a great foil and variation to Sting's. Most of his input are among my favourite Police songs.
Todd Rundgren went on to great fame in the 70s but his output in the 60s on the Nazz's 3 LPs are incredible. He was one of the best writers of the 60s in my opinion, truly underrated, like Gene Clarke of the Byrds.
As for Bruce Cockburn, there was a time when he was known as 'the last rock obscurity'. And with good reason. In my whole life I've only ever met one person {and known of two} that has heard anything by him, let alone heard of him. He's been writing great stuff since 1969 and has a truly diverse output, doing folk, stinging rock, reggae, jazzy stuff, spoken word etc and utilizing a vast array of instruments over the years. His songs are fantastic.