I'm looking to make a Nuke expression: I want the maximum value of an animation curve of another node
There's not a built in method for that, but this Python expression will return the highest keyframe in an animation curve:

max((key.y for key in nuke.toNode('Blur1')['size'].animation(0).keys()))


where Blur1 is the node and size is the knob.

Paste this into your expression editor, being sure to select the Py toggle.



This won't interpolate or extrapolate keys. It only iterates over every keyframe.

No guarantees on the speed of this either. It might bog down the node if you have a keyframe on every frame or something.