By far.
The Quarian Admirals know a war with the Reapers are coming, and yet they choose that exact moment to launch a risky and potentially disastrous invasion of Geth space. What is there to gain from capturing Rannoch, if the Reapers are going to annihilate everyone on that world shortly after you liberate it? That is easily the largest strategic blunder made by any character in the series.
Koris is the only one out of the bunch with a brain. Xen is completely mad, Shala'Raan is weak, and Han'Gerrel is a great blundering oaf.
*sigh* I really didn't want to get drawn into this...
Let's get one thing straight: the Migrant Fleet cannot contribute to the Reaper war effort in any way so long as their cargo holds are physically full with civilian housing. In their state before the Reaper war, they needed to stay close enough to a liveship for daily food deliveries, and were dependent on external infrastructure (which the Reapers quickly went about destroying) for basic survival. Logistics demand that they stay clumped together in a single mass which takes days to traverse a relay in order to keep everyone fed and protected. For their ships to be of use, they have to offload their civilians. That's non-negotiable.
There was no indication that negotiations between the Quarians and Geth would have borne fruit even if Legion hadn't severed comms before the Reapers invaded. Even if you bring Legion to the fleet, he says the Geth
aren't willing to negotiate unless certain nebulously-defined conditions are met. Every diplomatic mission sent into Geth space for the last three centuries was shot down on sight and no effort was made by the Geth to distance themselves from the Heretics' actions. Prior to meeting Legion, all the information available to Shepard says the Geth are categorically Reaper allies - simply put, any Shepard who promotes peace talks before meeting Legion is an idiot.
Come ME3, Legion severed comms, putting an end to any notions of peace. The Reapers have invaded the galaxy, tearing down even military superpowers like the Turians. Meanwhile, the Quarians are bobbing along in an aging, less-than-maneuverable fleet, knowing that one Reaper entering the system they're in and taking potshots at the Liveships would spell extinction. They had to act in order to survive. But what options are open to them?
Koris' stated plan was to hide in space, a state in which they cannot contribute to the Reaper war effort in any way ("We don't need a planet! The Migrant Fleet has survived for three centuries!" never mind it had eighty years left, tops, and the Reapers take centuries to sweep for survivors). Koris is a coward whose Reaper contingency plan would have resulted in the extinction of his species.
Gerrel and Xen, for all their faults, did what had to be done to ensure their species' survival while freeing up their fleet for the war effort at the same time. Rannoch is by no means immune to attack (nothing is) - once they've offloaded their civilians there, they're throwing in on the Crucible project like everyone else, depending on its success to save them. Would a peaceful resolution have been preferable? Of course. That was a failure on both sides of the table - Shepard's time would have been far better spent attempting mediation between them than jerking off in an Alliance holding cell.