The word that best completes the sentence is "fanatical."
The sentence would read: "Some of the team's fans were fanatical and spent incredible amounts of time and money following them around the country."
"Fanatical" describes someone who has an extreme enthusiasm or zeal, especially for a particular cause or hobby, which fits perfectly in the context of devoted sports fans.
The other words do not fit the context as well:
- "Fatuous" means silly or pointless, which does not convey the commitment of the fans.
- "Feline" relates to cats, which is irrelevant here.
- "Flagrant" means something that is conspicuously bad or offensive, which also does not capture the essence of devoted fans.
So, the correct answer is "fanatical."