Club soccer: Liverpool (x10), Chelsea, Real Madrid, Barcelona, Juventus, Bayern Munchen, Ajax, Feyenoord, Standard de Liège
International soccer: Spain, France, Belgium, Portugal, Denmark, Sweden, Norway, Brazil, Greece
MLB: Yankees, Red Sox, Cubs, Dodgers
NFL: Steelers, Ravens, Cowboys, Broncos
NBA: Lakers, Celtics, Heat
NHL: nobody in particular
NCAA F: Southern California, UCLA, Michigan, Florida State & the entire SEC except Vandy
NCAA B: UCLA, North Carolina, Duke
Ooh, and always route against Notre Dame.
NFL: Eagles, Redskins
MLB: White Sox, Mets
NBA: Lakers
NCAAF: Texas A&M, Kansas State, Oklahoma, Miami, Florida, SMU
MLB: Yankees, Braves
NFL: Cowboys (mild), Raiders
NBA: None, really. Maybe the Knicks
College FB : Miami, Georgia
College BB : Georgia, UNC
Joe
MLB: New York Yankees
NFL: New York Yankees
NBA: New York Yankees
NHL: Toronto Maple Leafs
Soccer: New York Yankees
Cricket: New York Yankees
Rugby: New York Yankees
NCAA: New York Yankees
What?
NBA: Lakers and Heat - I really dislike Kobe Bryan and Lebron James. They seem like babies that never grew up.
MLB: Yankees, if pressed.
NHL: No one really, but I guess the Avalanche if pressed.
NFL: No one
Tennis: Serena Williams - I’m embarrassed that she plays Fed Cup for the USA sometimes to the point of not wanting to ever win it with her on the team. Her tirade against the lines-woman was so bad, she should have been banned from the US Open the next year, but wasn’t.
I’ll add:
NCAA: Michigan State and Ohio State - I cheer against them always.
Cowboys
NY Giants
Yankees
Lakers
Duke
Steelers, Steelers, Steelers, Steelers, Steelers, Steelers, Steelers, Steelers, Duke, Louisville, NY Yankees, St Louis Cardinals.
NFL: Vikings, Cowboys. Once upon a time, I would have put the Bears on that list, but living in Chicago for 22 years may have mellowed me on that. 
MLB: Yankees
I follow the NHL and NCAA football, but I can’t get enough hate on for any particular team to list as a “least favorite”.
Seconded. And I don’t even care about football. It’s more due to the fans than the team itself - I’ve never seen a bigger collection of scumbags. I spent most of my life in the Bay Area, BTW.
Swindon Town - they are scum. Swindon’s most famous landmark is a roundabout - says it all really.
MLB: Yankees, White Sox
NFL: Lions
NBA: Lakers, Bulls
NHL: Avalanche
NCAAF: Ohio State, any Florida team
NCAAB: Duke, NC, any Florida team
Dread Pirate Jimbo has done all my thinking for me.
OH – except I have no animosity for the Maple Leafs. He can hate them all on his own.
Routing against Notre Dame is pretty easy as long as you avoid the Indiana Toll Road…
The Eagles above all else. The Cowboys and Chargers are on a much lower tier.
The Phillies and, somewhat less, the Braves.
Whenever the World Cup rolls around I root against Argentina. This stems from an Argentina-Nigeria game I attended when the cup came to the U.S. The Nigerian fans were impressively well-groomed, polite, etc. The Argentine fans, not so much.
I can’t tell if I hate or love that everyone hates the Steelers. It does give me a little warm feeling inside, so I guess I like it.
MLB: Yankees
NHL: Capitals
NBA: Is that still a sport? Why?
WCS: Germany, now South Africa
NFL: Baltimore. Just thinking of wearing one of their jerseys is nausiating.
NHL: Toronto, Philly, Boston
CFL: Toronto
NFL: Dallas, though I have no idea why.
NBA: Toronto
MLB: Toronto
I think there might be a common theme here…
Major league baseball: The Red Sox
NFL: The Eagles
College football: Oklahoma
NBA: Lakers
College basketball: Kentucky
NHL: Bruins