Accounts are sorted so that your Cross Save primary account displays first, then in the order they were last played. Usually the one at the top of the list should be the one you select.
Will the account I select affect the popularity ranks / theorycrafter recommendations I see?
No, these stats and recommendations are independent of what platform you play on or what input method you use.
The account I'm looking for isn't showing up. What gives?
A couple things to try:
Make sure the Bungie API isn't down by checking @BungieHelp
Make sure you signed into the correct Bungie.net account.
If the Bungie Tag you see to the left doesn't match the Bungie Tag on your character select screen in-game, you're probably logging in with the wrong account.