[Wrana]

Well, you can consider to take weighted mean, for example. I.e., if somebody had consistently won with the nation in question, his estimate of it could be taken, e.g., double before averaging. Or, say, one more time for each time he won in MP with it (which can be checked in Hall of Fame). This will make more experienced players' opinions count for more (but if there are significant discrepancies with other estimates, you can lower their "ease of learn/use" score).