Setting default value of a drop down menu in nuke with python

Try the following one. Use this code for nodes with specific names:

def removeOverride():
    nuke.toNode("Remove1")['operation'].setValue('keep')
    nuke.toNode("Remove1")['channels'].setValue('rgb')

nuke.addOnUserCreate(removeOverride(), nodeClass='Remove') 

# use UpperCamelCasing for any class name ('Blur','Read','Remove')

Also for any subsequent nodes created, use this code:

def removeOverride():
    for node in nuke.allNodes():
        if node.Class() == "Remove":
            node["operation"].setValue("keep")
            node["channels"].setValue("rgb")

removeOverride()
# OR
nuke.addOnUserCreate(removeOverride(), nodeClass='Remove')

And the last configuration using nuke.selectedNodes() and nuke.thisNode() methods is suitable for selected nodes in the DAG:

def removeOverride():

    thisNode = nuke.thisNode()
    rootDAG = nuke.root()

    with rootDAG:
        allNodesSelected = nuke.selectedNodes()
    try:
        allNodesSelected.remove(thisNode)
    except:
        pass

    for node in allNodesSelected:
        node["operation"].setValue("keep")
        node["channels"].setValue("rgb")

    return allNodesSelected

nuke.addOnUserCreate(removeOverride(), nodeClass='Remove')