I really like the head replacements from ACHAI studios. I got one for Optimus, Rodimus, Sandstorm and Kup. All look amazing. Probably the most dramatic difference besides Sandstorm, of course is Kup. Night and day.
In subreddit r/transformers
In a post titled:
A new head can make a figure so much cooler than it already is [NB-01 Upgrade for SS86 Optimus]