I just answer these things honestly: "gosh, ehm, I'm having a hard time thinking of a specific thing right now", and then turn around the question to answer something slightly different, for example a particular difficult bug that I might remember, or a somewhat (not necessarily "most") interesting or difficult thing I fixed recently (and is thus fresh in memory, stressing this is just something I did recently).

They don't really care what the most interesting bug is anyway, they just want to have a conversation surrounding bugs. You can also turn it to "well, in general [..]".

Also don't be afraid to say "ehm, let me think for a second", instead of feeling obliged to answer right away.