For NFL, my highest was just this year, the Eagles winning the Super Bowl. The low light has been the years of Bobby Hoying and Ty Detmer, as well as losing the first Super Bowl to the Pats and the consecutive NFC Championship games.
NCAA highlight is when PSU beat Florida State I think in triple overtime, and the lowlight being the Sandusky stuff and JoePa being who he was. From a basketball standpoint, watching my area's local college, Bucknell, upset the 3-seed Kansas in the Tourney back in I think 06 or 07 was amazing to watch.
I only became a Liverpool fan in the last 10-ish years so many lowlights. My first memory of my fandom was the Riise own goal against Chelsea in the first leg to give them an equalizer at the death; selling Torres who I loved; the Europa League Final loss; and of course, the slip. My highlights are the comeback against Dortmund, the FSG takeover saving the club; and the game against Man City in the 13/14 season. I truly thought we were winning the title after that match.
For the USMNT, the bullshit against Germany in the 02WC, going to my first qualifier this year and watching them lose to Costa Rica, the way we fell apart to Mexico in the 2011 Gold Cup, and obviously, the Trinidad game and failing to qualify for the WC. Highlights are Benny's screamer to win the 07 Gold Cup and that Landon Donovan goal against Algeria in the 2010 WC. That was magical.