I wouldn't go so far to call it the best, but it is well casted and well acted. Character continuity from the series is pretty thin and the introduction of one-shot characters like Lilly who encompasses most of the dramatic scenes does rob our main cast for potential growth. Not to say Lilly's scenes weren't any good, but it is what it is. A character who we will never see again who only serves to cause growth in the main character.
But the biggest problem with First Contact is the introduction of the Borg Queen. For a race of beings that were once depicted as being made up of collective minds who didn't have a single leader I thought was unique and also horrifying. Giving them a leader who doesn't really act like a collective of minds since has to give commands to her drones just makes the Borg out to be her minions and not a collective. It also gives us something to familiarize and be comfortable with since if you kill her all the other drones will just die. Unfortunately the producers and writers of Voyager decided to latch onto the Borg Queen concept and take it to levels that ended leaving the Borg as one of the weakest villains ever to grace Star Trek.
For me, the best part of First Contact is Jerry/Joel Goldsmith's music. Their work on the film is just fantastic to listen to.