What is the most powerful move that hits both opponents in a double battle?

Question

and which Pokemon can learn it?

Answer 1

If the user has 100% of it's HP left, Water Spout and Eruption have the highest, at 150 Base Power.

Pokemon that learn Eruption

Cyndaquil
Quilava
Typhlosion
Entei
Camerupt
Groudon
Torkoal (breeding)
Heatran (event)

Pokemon that learn Water Spout

Wailmer
Wailord
Kyogre
Frillish
Jellicent (both forms)
Squirtle (breeding)
Remoraid (breeding)

Otherwise, it's Blizzard, with 120 Base Power.

http://bulbapedia.bulbagarden.net/wiki/Double_Battle#Effects_on_moves
See All foes.

Answer 2

Explosion would be, at 250 BP.

Here is a list of Pokemon that can learn it.

Answer 3

Its Blizzard. There are too many to list that can learn it so look here