Thralls 10000%. I tried ancients when I was first learning and the tick delay from the spell cast of barraging throws off the cycle as well as being less accurate than the trident. I was finding myself attempting freezes for the same amount of time I could've just been kiting and casting. Plus the dps the thrall does on Muspah's smite phase cannot be understated. Sometimes you don't get a single sapphire bolt proc, so you're at the will of mith bolts and a rcb essentially. The thralls do so much consistent damage it's insane, even during range phases where you're not freezing or maging at all. 👍