i think otb or online federations with a large amount of games having been played by a player (hundreds or thousands) arguably might give the closest because of rds. but overall i think chess.com's are the closest of any non federation to a player's actual playing strength if a player's not been rding all over the place and doesn't have a fide,ecf or uscf rating. and i tend to look to it (my standard/blitz) to measure how much i've been improving/how strong i'm getting because my rd's normal here and my rating also arguably is in comparsion to otb play whereas over on lichess i'm above 2100 in some of my ratings lol which i most definitely am not, and on foa i tend to be about 300-400 points lower then over here.
What type of chess provides the most accurate rating?
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I think mentor is inflated and TT is the most accurate