Best nuts but not the ultimate biscuit (5,7)

I believe the answer is:

cream cracker

I'm a little stuck... Click here to teach me more about this clue!

'the ultimate biscuit' is the definition.
'cream cracker' can be an answer for 'biscuit' (I've seen this before). I am not certain of the 'the ultimate' bit.

'best nuts but' is the wordplay.
'best' becomes 'cream' (both can mean the elite).
'nuts but' becomes 'cracker' (I can't explain this - if you can you should give a lot more credence to this answer).
'cream'+'cracker'='CREAM CRACKER'

'not' is the link.

Can you help me to learn more?

(Another definition for cream cracker that I've seen is " Biscuit".)

I've seen this clue in The Sunday Times and The Times.

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