I always thought
soo-i-sine or soo-i-coon
I'm pretty sure it is Swee-cune.
I agree with what most people have said, "Swee-coon." Then again... I probably can't pronounce half of the pokemon names how they were meant to sound. xD
According to an official Pokemon handbook, the Nintendo-approved way of pronouncing this is 'SWEE-koon'