Either Pex regular with the crimp rings or a sharkbite fitting. The sharkbite fittings are more expensive, but for the one connector and the ability to just press the tool in and pull it of to press a new tank fitting on in the future might make the change out easier.
If it were me, I would do the pex with crimp ring. You can always just cut the pex off and cut the rings off you can install a new short piece of pex with crimp rings.
I don't trust the sharkbite. They are suppose to be ok, but a long term water fitting releying on a rubber gasket worries me. They have only been on the market for about 15 years.