Who is Supreme, Krishna or Shiva?
The Srimad Bhagavatam 1.3.28 says:
"All of the above-mentioned incarnations are either plenary portions or portions of the plenary portions of the Lord, but Lord Sri Krishna is the original Personality of Godhead. All of them appear on planets whenever there is a disturbance created by the atheists. The Lord incarnates to protect the theists."
Also Lord Brahma, in the first verse of the Bramha Samhita says:
"Krishna, who is known as Govinda is the Supreme Godhead. He has an eternal blissful spiritual body. He is the origin of all. He has no other origin and He is the prime cause of all causes."
Lord Shiva is described in the Brahma-samhita to be like curd or yogurt. Curd is not different from milk. Since milk is transformed into curd, in one sense curd is also milk. Similarly, Shiva is in one sense the Supreme Personality of Godhead, but in another sense he is not, just as curd is milk yet we have to distinguish between the two.
Lord Brahma also explains that many candles lit by a single candle may all look similar, yet one candle remains the original. Krishna is like the original candle, because He is the source of Lord Siva and all other spiritual and material energies.