In terms of actual physical strenght, Tulkas is the man!
However, there is much more to the power of the Valar than simple physical strength. Each was supreme in his or her own bailiwick, I suppose (for example, I don't think any of the Valar could have withstood Ulmo if they were caught in the middle of the ocean).
Their relative rank is listed in the Silmarillion.
- Cirdan / Maglor68
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