I put root in quotations because I'm not sure if it's used outside of North America or not. Anyway it means a team you cheer for. Here are mine:

Calgary Flames/Detroit Red Wings (NHL): some people treat cheering for two teams like polygamy, but this an exception I can assure you. I grew up with a family of Red Wings fans so I have a soft spot for them, but I was also a Flames fan because their logo was a Flaming C (as a kid I thought it was cool because my first name starts with a C).

Detroit Tigers (MLB): For some reason when I was a kid I liked the White Sox, but now I stick with the Detroit team.

Detroit Lions (NFL): For some reason when I was a kid I liked the Dolphins, but now I stick with the Detroit team.

Detroit Pistons (NBA): ...

Middlesbrough (Club football): When I first started following the sport closely in 2011, I watched a documentary and Middlesbrough won me over. Unfortunately they've only been in the Premier League one season since then. Maybe of Detroit got an MLS team they would be my main allegiance.

Richmond Tigers (AFL): I got into Australian rules football last year right before the season started and picked Richmond partly because they hadn't won anything since the 1980s. But then Richmond were champions last year.

For NFL- I root for 2 teams because some AFC and NFC teams rarely have matches against each other. The 2 teams being SF 49ers and PIT Steelers. The dream of course is a super bowl between the 2. The 2 teams are actually 11-10 in favor of 49ers, so 21 games in over 67 years. About 1 game per 3 years, so I think being a fan of 2 teams is quite reasonable otherwise there will be many games that I just don't care about.

MLB- I like the Cubs.

Brisbane Broncos- NRL my hometown team.
Most popular NRL and even rugby league team, most watched team on television and live- over 30k average whilst Sydney teams average 5-10k
7 grand finals, 6 wins- 92.93.97.98.00&06- W, ‘15-L by one point, all under the same coach. He also was the first coach for the club from 1988(on his second tenure at the club due to moving down south to St George and Newcastle for three years each)

I don’t watch anything else but I do enjoy watching NHL on Foxtel every now and then when I have my dinner/coffee breaks at work.
As for AFL, I don’t watch it here as AFL is pretty much nonexistent in Queensland(rugby league mad state) but when I move down to Melbourne I’ll start watching games more often. The teams in Queensland are just absolutely rubbish( Brisbane Lions was the premier team back in 01-04 when they appeared in 4 straight grand finals with 3 wins a row and the last one was a big defeat apparently)

Liverpool FC

The Swedish Bikini Team.

Chelsea FC

Complexity, Galeforce, Fnatic, nV

NHL - Detroit Red Wings
NFL - Green Bay Packers/Detroit Lions
MLB - San Francisco Giants/Detroit Tigers
NBA - Miami Heat/Detroit Pistons

Chicago Cubs - Always been my favorite team, and in some odd fashion the fact that they hadn't won a title in so long up until 2016 actually drew me more to them and made rooting for them more fun. Now I just want to see them build a dynasty and make up for that huge world series gap by winning a few more championships.

Chicago Bulls - Of course, the Jordan era is what really won me over, and seeing all those games in the United center both as a kid, and more recently watching the Derrick Rose Bulls dominate most of the east. That was, until they ran into the brick wall that was LeBron's teams and D Rose had his career altering injury :(. Still, the Bulls are probably my second favorite team as I love basketball, probably even more than baseball, but between the Bulls sucking now and the NBA being heavily influenced by officiating, it's tough for me to really get into them these days.

Blackhawks - Have great memories watching many a playoff hockey game with my friends back in the 2009-2015 post-cap dynasty era with Kane, Toews, Hossa, Seabrook, etc. Not a HUGE hockey guy though I do love the excitement and intensity in the playoffs in particular. Hoping the Hawks can recapture some of their former glory and reload next year, though unfortunately they're starting to age.

Bears - more of a casual fan when it comes to the Bears as I'm not really a football fan. Though it would be cool to see them win a superbowl as they're now the only Chicago sports team not to win a championship in my lifetime (just barely missing the 85 superbowl as I was born in late '86)..

Far more recently, I've also been into watching the Overwatch league (if esports count) and root for a couple of teams there -

Houston Outlaws - They've got a prett solid Zen and D.va player, who are two of my favorite characters, Jake's also fun to watch with the Junkrat and Linkzer's a beast with Widow sniping. I also learned that the company that founded them, Optic Gaming, is actually based in Chicago too, only moving to Texas in the last few years. Of course if Chicago ever gets a real OW team I'll root for them over Houston but for now, Outlaws are my team.

Shanghai Dragons - Like the fact that they have the only female in the league in Geguri, who comes with an inspirational story. And of course, I'm just pulling for them to get a win as they're the only winless team still in the league lol.

Football (Soccer): Atlético Nacional (Medellín, Colombia), Chelsea FC (England)

NBA: Boston Celtics

NHL: I stopped following the league because I did not have time anymore but at the time I liked the Anaheim Ducks.

NFL: I started following the league in 2012 and the Dallas Cowboys in particular (the reason was a bit random, at that time I was deciding which team to follow and at that moment I was watching "Lost" and mentioned them in series and I just thought that it was the destination xD). Anyway, after the retirement of Tony Romo it was very difficult for me to maintain interest in the team and the release of Dez Bryant was enough for me (Romo and Dez were the players that most attracted my attention and my favorites when I started to follow the Dallas Cowboys). So I do not see the NFL anymore.