You actually answer the question yourself.
What is the best name for this map?
You want to map your map to a name, so you say 'name for map' !!
That should be the naming convention, in my opinion: valueForKey.
With the other suggestions keyToValue and valueByKey, I feel you need to add the word Map at the end, like this: keyToValueMap, valueByKeyMap. When you use For it's apparent from the language that it is a mapping.
