If I have a platoon of tanks and a platoon of pc's, two tanks lead the column, while two tanks trail the column.
If I have scouts, the scouts lead with one tank attached, while the main column stays a ways behind.
If my force is mostly tanks with just a platoon of infantry for support, I lead with tanks in all situations in at least platoon strength.
If it's more like a company of infantry with an attached tank platoon, I usually lead with the tank platoon followed by either mounted or dismounted infantry platoon except in areas where enemy AT assets are likely to be present, in which case I'll lead with a dismounted infantry platoon, and have the tanks stay one or two steps behind as support.
You almost NEVER want to lead your column with mounted infantry unless speed is the sole factor behind your movement. Infantry mounted in pc's or riding tanks are far too vulnerable to literally anything the enemy might have. In fact, I would never have infantry ride tanks unless it was a completely safe environment, and I would dismount at even the first whiff of anything larger than a pistol pointed my way.
I really need to get around to making that "Quick Guide to Mechanized and Armored Infantry" video.