First Contact was an amazing movie. It had flaws, but it succeeded for the first time (for me, at least) at translating the in-universe fear the characters had of the Borg to the audience. There's a difference between routine invincibility and scary. Throughout TNG, the Borg were merely another overpowered threat enemy that you had to be tricksy to beat. But in FC, they were terrifying, and I think the production team did an outstanding job. At times, it was like watching the first Alien.
I'm of the opinion that time travel stories are never good, at least when time travel is the focus rather than just an excuse to get to a fun setting.
Since time travel seems to be a physical impossibility, essentially any set of rules you create for it are just as valid/nonsensical. So you spend a bunch of time dealing with technobabble and paradox talk that has no hope of making sense outside of whatever the author needs for the story.