I find that there are parts of each soundtrack that I like better than others, so it's hard to say best "soundtrack".
However, I'll do my best:
1. TMP. Truly classic masterpiece.
2. TWOK. Emotional and exciting for me.
3. TFF. Not only continuing Goldsmith's brilliant work, but adds some wonderful stuff, including the Yosemite stuff, and "A Busy Man."
4. First Contact. Actually, it's my favorite Star Trek main theme. However, some of the tracks I find a bit monotonous. Still, there's some great cues.
5. TVH. Fun and new. I liked how they included more contemporary sounds to go along with the time travel to the primitive 1980s.
I'll also mention that even though the film isn't widely regarded, I think the music from Insurrection was beautiful.