My best upgrade was on Carnival Paradise in 2004. We recieved an upgrade from 6A gty to a 11 or 12 (I don't know the difference) balcony suite. It was so great and will probably never happen again.
I think it happened because I have medicine that requires a fridge and the fantasy class ships at this time only had fridges in the suites (no 8As). The ship must not have been filled so instead of bring in a fridge to a 6A they moved me to the fridge. A year later we took another cruise on a fantasy class ship and they squeezed the fridge into the 6A room. My hubby was bummed he wanted another upgrade.