A few of my favourites:
Led Zeppelin - "Thank You"
Rush - "Nobody's Hero" and "Afterimage"
Journey - "Winds of March" (old Santana-inspired Journey, long before Steve Perry turned this talented band to cheesy shit)
Grateful Dead - "I will take you home"
Pretenders - "Back on the chain gang"
The Who - "Behind blue eyes"
Fleetwood Mac - "These Strange Times"
I dunno if any of these are "ballads" in traditional sense but they're all beautiful, sad - and still very powerful songs. Maybe Eric Clapton's "Tears in Heaven" is a bit too much, but it's a good song about a sad tragedy.