Betsy and Orc, the issue with them is that you have to do the event again from the start, for a summoner this is okay as this just gives your more sentries to use, but other classes it's all about betsy weapons which are pretty good, problem is getting the drop you wanted.
Other is the blood moon mini-boss I forgot the name, mostly because Blood Moon isn't easy to force trigger as the other events, so trying to get sanguine staff it quite the pain.