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 is excessively enthusiastic or devoted, which fits the context of fans devoted to a team. The other options do not convey that meaning:
- "Feline" relates to cats or can describe something cat-like.
- "Flagrant" means something that is obviously bad or reprehensible.
- "Fatuous" means silly or foolish.
Therefore, "fanatical" is the most appropriate choice.