NFL: Patriots
NBA: Bulls
Soccer: Tottenham Hotspur
Soccer club national team: England, USMNT
Being someone who lives in the UK, my dad took me to my first soccer game when I was a teenager to watch Tottenham and Gary Lineker play and was a fan ever since. I sometimes curse him why he didn't take me to watch a team like Man Utd instead...
Patriots was a funny story. My friend brought some tickets a few years ago to a NFL game being played at Wembley and asked me to go along with him. I didn't know much about American football but went anyway because it sounded like it would be fun. Brady and Gronk crushed the Rams by 40ish points that day. I thought it was a pretty interesting spectacle and decided to follow the sport. Because I had no affinity to any of the teams, I had to choose one at random. I chose Patriots because a) They had 'England' in their name, b) Their logo was pretty cool and c) I liked the colour of their uniforms as it was very similar to Tottenhams'. Turns out it was a pretty good choice heh.
Bulls because of Jordan and Pippen. Although I don't follow NBA that closely.
Soccer national teams I support England but it's been an abusive relationship since they are so shit. Every time they built my hopes up before major international tournaments, they crush them mercilessly. I have since switched my alligence to USMNT whenever they play in major tournaments since they are actually entertaining to watch rather than the turgid crap England plays.