blue paradise gourami (m. opercularis), if obtained without dropsy or internal parasites, and fed a mixed diet to avoid constipation, are unbelievably resilient.
I'd say Gouramis or plecos. I once had a heater stop working and the tank dropped to below 70 before I noticed and their both still kickin' two years later.
Dwarf Honey gouramis. I had a friend once who completely forgot about their tank, the temp dropped below 60 degrees, they were left unfed, and the nitrite rose above .5 ppm. (Bacteria all died.) This lasted for a month, until I resuced them. I promptly took them home and threw them into my tank. They're still alive. Did I mention that he never did a water change for as long as he owned the tank? (Nitrates were well over 160 ppm when I rescued them.) Bought them a few friends, and now I have a happy school of five honey gouramis breeding in my 29 gallon.
They're good with just about any fish. Different species though: neon blues are colisa lalia, honey gouramis are colisa chuna. Both are very nice though. IMHO the honey gourami is more peaceful and hardier. I found that they're very close to indestructable.
I'd have to go with plecos. The only part of them that's not reinforced with armor plating is their belly. They rapidly grow to multiple feet. They keep the tank clean of algae. They can also live out of water a heck of a lot longer than most fish if they happen to jump out.
My leopard sailfin survived the plague in my tank, though it may still be going on. Somebody (probably the spotted pike cichlid that died) took a huge chunk out of my plecos sailfin, yet it grew back very rapidly.
The only things against them are they come out at night, and mentioned above, outgrow most tanks.