Reading through a few posts lately, and I'm not sure if some people aren't misunderstanding the Flex position.
Some posts read like some people think the Flex position replaces your 22nd/lowest score.
If so, you are looking at it all wrong. The Flex position is just an "extra" player, giving you 23 players on the field. You score your best 22 of those 23 each week. The Flex has the advantage of being a free pick position wise, and the disadvantage of having no bench cover. As it has no bench cover, it is generally best you put a player there that plays early in the Round, so you are not caught out if there are any late changes that you can't cover.
If you are playing the 3 Ruck gambit through your Flex position (hopefully with a cheap R/F at R3, to allow a Fwd bench player to cover one of your 3 Rucks if they are missing a game one week), then the best Ruck to put in your Flex position is the one of those 3 that plays first in the Round.