I got a purple drop from Harley in the solo challenge. If I remember correctly it's Harley's jester hat and it has really some good stats. The style you unlock is different from the other clown/joker style. Sometimes I will use it as my head armor because it looks so weird and cool.
After having done most of the solo challenges 2-3 times up to this point, that's all I've been able to get so far though. --- You think you know, but you don't know anything at all.
the challenge missions are just missions you have already done (for heroes its fighting ivey, harley, dr. psycho, faust, trigon and spectre again) but on hard. all enemies are lvl30. these are easily doable once youve done a few and get the good equips.