Benjamin Olson
Hero
Any rogue is broken once you use the optional flanking rule. It's an optional rule for a reason: it was undercooked and doesn't play well, but the designers knew that flanking should provide some sort of benefit and so they had to at least include the system they never quite figured out in the DMG.There are few builds more broken than an elven swashbuckler rogue with booming blade and trivantage at a table that uses the optional flanking rule.
At my table you get an extra d4 on the attack roll if you flank someone, because unlike the 5e designers I don't have strong compunctions about using something other than advantage. Works pretty good.