grammatical number - "First part are [plural]", or "first part is [plural]"?

I'm currently writing my master thesis on Bitcoin and I'm not sure which version of this sentence is correct:

"The first most important part of the Bitcoin infrastructure are all applications that communicate with the Network."

"The first most important part of the Bitcoin infrastructure is all applications that communicate with the Network."

Or perhaps both of them are wrong?

EDIT:

Finished sentences.