I love that water drop effect on the post-6th Gen Calm Mind, but my all-time fave move animation is a tie between Water Pulse and Surf. Water-type moves always have neat animations, and I love how Surf is this giant wave crashing down on the foe, while Water Pulse's animation in B2/W2 was definitely its most gorgeous.
Unfortunately I hated Surf's 6th Gen animation so I was glad 7th Gen changed it back into the wave.