What is the right adjective to describe meat?
The sentence is: "We stopped eating our sandwiches when we realised the meat was ... " What adjective should I use to say that the meat was spoiled/smelled bad?
Rotten/rank/foul or any other adjectives?

Question

Answer

Most English speakers would describe meat that had a bad TASTE by saying, 
"the meat has gone bad",  "the meat has gone off" or "the meat tastes rancid".
The word "rotten" would be used for food is in a noticeable STATE OF DECAY.
Words such as putrid, foul and rank would be used to describe a bad SMELL.

Hope this helps

Answer

I agree with Naomi’s answer, but I would add that your suggestion of “was spoiled” would sound very natural and that “rancid” is fairly uncommon in U.S. English.