One thing that I saw claimed, I think, on a TV program a few years back is that a lot of the roast chicken/smoky bacon/beef/cheese and onion type flaours are actually extremely hard to identify blind, and fairly hard to tell apart even in comparison. I did try it with a few and I certainly had difficulty telling what was what without the packet.
Although I already voted for Marmite, cheese and onion comes a strong second place. I used to really love the KP ones in yellow packets. The Seabrooks cheese and onion flavouring reminds me of those somewhat.