I looked a bit for the “featured” ranking algorithm but gave up looking for now.
Using Topic Ratings Plugin as an example
The avatars are
angus - Original Poster, Most Recent Poster (28)
fackelwind - Frequent Poster (10)
zogstrip - Frequent Poster (2) [2/ 1]
envieme - Frequent Poster (4) [1]
repz - Frequent Poster (next to last)
So it looks like the number of posts in the thread is important, as are the “last” posts.
i.e. a combination of both post count and post sequence.
The odd man out here is zogstrip who has only 2 posts in the topic yet is before envieme who has 4 posts in the topic.
Reason? My guess would be that Staff or maybe Trust Level are weighted more.
I suspected the number of Likes might be a factor, but if it is it doesn’t look to be in this case.