I can only pick one?! Hmm... Well, my favorites are the deadlift for time, tire flip, McGlashen Stones and Farmer's Walks.
However, going for only one of those, I have to pick Farmer's Walks. They're awesome as a strongman lift, but practical for every lifter, and don't require any special equipment - just DBs or some Ez Curl Bars.
As a strongman event, walking while holding 400+ lbs is just freaking impressive, given that most guys can't even DL that. And considering that the event can be done for time or distance, it's quite versatile in the torture it offers to competitors. It's certainly not as dramatic as, say, the log press, caber toss, or car pull, but the sheer tenacity and strength endurance that Farmer's Walks require makes it a favorite.
For the average trainee, who isn't going to be competing in a strongman comp, Farmer's Walks still rock. Like HIIT? Try speed-walking with a 125lb DB in one hand, without allowing your core to bend or other arm to extend, and tell me that doesn't make your heart rate skyrocket and BF% drop! Wanna improve your deadlift? The extended grip-work and core activation of FWs will go a long way to get you pulling new PRs. And improved grip translates to bigger forearms and I mean, c'mon, everybody wants forearms like Irish Cannon's.
Edit: I don't even know if I'm qualified to win this, but I don't care - I like strongman comps! Sometimes I feel I missed my calling and should just go pick up super heavy stuff...