TWOK is still my favorite of all the ST movies. TSFS is kind of "part 2" of the story. If I hadn't enjoyed TWOK as much as I do, I might give TSFS a higher rating. I like it, but it doesn't measure up to TWOK in my opinion.
I think TWOK is quite self-contained, despite all the sequel hooks right near the end. It can definitely be watched as it's own standalone thing. TSFS on the other hand is very much tied to the events of the movie before it, and by extension it still hasn't wrapped things up by the end, necessitating a further sequel to finish off the story.
On some levels TSFS and TVH are more closely linked than TWOK and TSFS, even though TSFS is a direct follow-up to TWOK.