Most underrated instrument in a song IYO?

Great call.

Although Ray Manzarek is far from my favorite musician, his organ solo in Light My Fire still makes the hair stand on end.

Another example: Doug Ingle, Iron Butterfly. There's maybe 3 songs in the IB collection that I like at all, but his organ solo in I-G-D-V kicks arse. It just does.

Jon Lord on Mandrake Root. Same thing, still jacks my bp after listening to it almost 50 years.

Organists, you rock.

Harmonica.

It's a tiny instrument and does an excellent job complementing a number of blues & folk songs.

Neil Young and Bob Dylan were both great harmonica players.

I agree with you. Organs are awesome. The last 3 or 4 songs I've written I've used an organ for the bass parts. I love the way they sound.

Rory Gallagher's sitar on Philby

Solos at 2:00 and 3:05

accordion

I don't know if it's considered underrated, but the mandolin was the first to come to mind.

More you-know-what. No not BOC; "Incense and Peppermint"/Strawberry Alarmclock:
LINK

Bagpipes

"Gotta have more cowbell baby"

Mandolin