What is the correct way to connect two adjacent panels?

I am going to be installing three panels next to each other. Two of them will by side-by-side, and a third one will be vertically above the one on the left. (To clarify, it will be a load center on the right, an AC disconnect on the left, and a meter socket above the disconnect). This will be outside my house. I have confused myself looking at the different ways I can connect these panels together.

For the two horizontal panels, I seem to have these options:

• A rigid nipple and two locknuts. Doesn’t seem to be watertight since there’s nothing pressing against the panel on the outside to form a seal.
• A close rigid nipple and two watertight conduit hubs.
• A rigid coupling and two chase nipples on the inside. Seems to be a nice watertight option, but unclear if the chase nipples have the "teeth" that bite into the panels to form the ground connection.
• Two PVC male adapters, a short PVC conduit section, locknuts and bushing. Will probably require a ground wire to be run to keep the ground connection.
• Rigid Offset nipple with locknuts and bushings. These seem to have a flange that presses against the panels on the outside.

As for the two vertical panels, I have a threaded hub on top of the disconnect. I was going to use a rigid nipple between the two, but it seems the same problem as above — the nipple doesn’t have anything to press against the bottom of the meter base on the outside to form a watertight seal. Is the right thing to use here a rigid nipple and then a water-tight conduit hub into the meter base?

A similar question — for the side-by-side case, what’s the normal way to make this connection when indoors? I have previously used a conduit between both panels and just a locknut on the inside of either panel. The tightness of the locknuts on either side press the nuts against the inside of both panels. Is that a code violation or is it OK?