I cast Nekusar.
Reasoning:
Considering just me: During my next turn I will be able to cast
Windfall and
Chaos Warp.
Windfall with Nekusar is an extra 4-6 damage per player assuming I remove
Spirit of the Labyrinth.
Mayael isn't a real threat. Fat
angel on the table, lots of mana but their strategy is probably to get a lot of fatties out. They're probably going at the Breya player if I keep my mouth shut. They've also missed a land drop so that pushes them down on the threat list.
Alesha's
Spirit of the Labyrinth is a problem but right now it actually protects me because my opponents don't get the extra card draw. Then again I have no idea what other kind of creatures there are so I'd prefer to wait.
Breya player is an obvious problem. Let's say there are a generous six tutors that find
Pili-Pala plus
Pili-Pala the card so a 7/89=8% chance to draw something that leads into
Pili-Pala. 8% is a risk I'm willing to take. If I remove
Spirit of the Labyrinth this chance goes up to 15% because of the extra card draw. I'm assuming that Breya doesn't have tutor in their hand already, which, of course is not the correct assumption.
Mayael is probably busy finding mana or fatties and won't really care if Nekusar is out. If anything they'll probably hope to get extra draw and hit a land. Alesha same thing, doesn't really make their play more difficult. Breya has probably a vested interest in keeping Nekusar
alive. More draw, more
Pili-Pala. Nekusar's win condition seems to be life loss so they probably count Nekusar as extra draw because they're all at 40 life. All in all there's no reason I'd lose my 5-mana investment during this round.
Finally let's see how likely it is that someone else removes
Grand Architect. Mayael might carry creature removal. All Naya colours can do this, meaning
Grand Architect is likely to be removed by Mayael since they also see the threat. Alesha also has access to good creature removal and can have a hate bear for this situation, something like
Phyrexian Revoker comes to mind. Never played Alesha so difficult to gauge but I'm sure there are other options, too. All in all there's a really good chance that either Mayael or Alesha will remove
Grand Architect before it gets back to me.
Nobody is interested in removing
Spirit of the Labyrinth (maybe Breya?) so that's probably something I'll have to do myself with
Chaos Warp.
Recap: cast Nekusar, wait, reassess situation next turn. Nobody is going to win this round,
Grand Architect might get removed. I'm basically back in the same situation but I've got Nekusar out and I know more about the board state.
EDIT:Chaos Orb =>
Chaos Warp (derp)