Castform.

I am true master of sky,
Even though I cannot fly.

It can't fly, but it certainly does control the weather, ie, the sky.

Thru rain and sleet, but not dead of night
I can surely put up a fight.

Rain and hail both give Castform its other forms, but night doesn't give it any bonuses.

Only few have more control than me
Of my true calling, you see.

The only more powerful weather-related Pokemon are Kyogre, Groudon, and Rayquaza, I believe.

My power has various types,
Though physical force is out of sight.

Castform can change its' form, and thus its' 'power', depending on the weather. However, its physical stats, as I recall, are terrible.