Marfitalu: It's an interesting question.

Any qualification has to include a range of games. A player specialising in one area cannot be the best all-round player by definition. (Though this leads to the idea of best players within the different categories - the best chess, checkers, line games, gammons, boats and pond players).

BKR is of no use in judging because a BKR in one game is incomparable with any other. (The Top 50 Average BKR list is practically meaningless). Using the standard deviation of BKRs would be possible but wouldn't be readily understandable to many people. (Try explaining it in one simple sentence).

Ranking is of some use but a rank of 22 in a field of 52 players is hardly special. This suggests relative position - a player's score for a game would be the percentage of the playing population below them in rank. Only the rankings of established players should be considered. It may even be prudent to only count those with, say, 50 matches under their belt.

The best all-rounder has to have played games in all categories (or maybe all but one) and I would suggest that the top three/four/five percentages be taken from a category. This would allow the evaluation to concentrate on a person's strengths, eg their 5 best chesses scores, their 3 best checkers, 5 lines, 3 gammons, etc.

All these scores (percentages) would be summed and averaged giving the player an overall score.

I haven't looked at many player's finished games lists but Pedro Martínez strikes me as a strong contender. He has only one ranking not within the top 100! and plenty of rankings within the top 20. In chess he's got a 1, a 4, 6, 10 and 11; in checkers he's got a 2, 3 and 4; in gammons he's got 16, 17 and 25 (a bit more work there Pedro, lol). Line games give him 6, 9, 9, 9 and 11; in boats it's 1, 6 and 10 and in miscellaneous he's got 3, 5, 7, 11 and 14. All very impressive.

Those are raw rankings. I haven't converted any of them to percentages (too much work, lol - perhaps you can do it Pedro?), so maybe some of those rankings aren't as valuable as might seem but it's got to be a high overall score.

Another strong contender is oliottavio. He's very strong in chess but under-represented in checkers (only two variants played) and weaker in backgammon. Like Pedro he's got plenty of top 20 rankings.

Another possibility is Matarilevich. He's the top man in ponds, strong in chess, only two variants of checkers (and that's why dropping one category should be allowed), but weak in backgammon and boats.