Jake <- Flushing 17,000 unslaved Yeerks
Edit:
Actually, I sympathized with him, and even thought it was completely justified. I also thought he had every right to enjoy it. It's amazing, but I can't really think of a Dethroning Moment of Suck for Jake.
For me that was the most disgusted I ever got with Jake. How many Yeerks in that pool were with the Yeerk Peace Movement? Sure he regrets the decision later but 17,000 unarmed lives were lost on a whim. Do you know how many 17,000 is? 9/11 cost 3000 lives? That's more than 5, 9/11s for a distraction!!
I only wrote the one line (unedited) because for the others nothing stands out quite as much as Jake's decision.
Another Jake moment of suck: The Conspiracy: Jake failing to take advantage of a golden opportunity to free his brother Tom, who was going to be alone with him and his parents for FOUR DAYS! I gather that the Yeerks were expecting Tom to die, so Jake could have arranged for Tom to be sent to the Hork-Bajir valley for de-Yeerking, and faked his death! When Tom's leg got broken, the Animorphs did a suck moment when they had the Chee take Tom back to town when they could have had Tom taken to the HB valley, and had the Chee fake a medivac helicopter crash.
Cassie suck: Allowing Tom to take the morphing cube.