Definitely Oat.
There are brands that make some specifically to emulate the flavour and texture of cow milk, often named something along the lines of "not milk".
Alternatively "Barista" variants often have a slightly raised fat content and added soy or other protein to improve foamability.

Cheapest option is to make it yourself: ~90g oats per 1000ml water

• soak and rinse oats
• blend oats, pinch of salt and water
  • don't blend too long, the mix gets a bit gluey otherwise
• strain through a cheese cloth or similar
• leave in the fridge overnight

Optional stuff:
add ~2 dates to the blender for a bit of sweetness
you can experiment with adding some cashews to the blend, or ~2 tbsp of neutral tasting oil to the blender for creamier consistency.
I heard xanthan gum can also be added to thicken the mix.