I've been farming story bosses primarily as they seem to have a better drop rate than Taken Champions and strike bosses for me. 
I farmed Primus Ta'aun and the other ultras when Regicide was the daily heroic. Nothing unusual dropped. I think I picked up No Backup Plans and an Alpha Lupi (hunter). 
The Gatelord on 240+ has been most generous to me. I picked up the 4th Horseman and Telesto from him. 
I thought about trying the Court of Oryx, but he last few times I've played, it was an over the top rapid fire Battle Royal. It's fun if there are a bunch of people doing it.
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	I tried ToO and it didn't work. Nothing dropped and and it kept the buff active. Tried tier one through three.
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	Side note, doesn't work on Skolas on story mode, yeah he doesn't really "die". It doesn't work on Ultra Devil Walkers either. The buff remains on both bosses.
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	It works for each of the shield brothers. I got an exotic off the first one killed, and an exotic off the second one too. Very lucky day for me
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	Cool. I haven't been so lucky. Regicide has four ultras, and I've only had a few drops. One thing I have noticed is that the Gate Lord at 240+ has dropped several purple engrams for me. t has dropped purple and yellow engrams at the same time. Time, that can't be right.
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	Same with psions aka the flayers
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	Good to know. I heart The Court when there are a bunch of players hanging around. It's like the Crucible for PvE. Elemental shields? Fill them full of bullets.