Won't mention the obvious one, but USO '15 F was an absolutely disgusting choke by Federer. Should have won that match in 3, max 4 sets, ended up losing in 4. It was pathetic, wasting chance after chance.
However my top 2 are Ljubicic losing in Madrid F 2005 to Nadal in the 5th set TB after completely destroying him in the first 2 sets and being a break up in the 5th, and losing to Baghdatis in 5 sets in AO QF 2006, his best chance for a Slam final.
We have a real contender in Rome right now. Shitty nextgen prospect that some still think will win a slam one day (LOL!) vs the number six in the world who maybe would have been the number 60 15-20 years ago judging from quality only. Strong era.