I am curious: what would you have done if you were the Sultai player picking reanimate targets here? I know that my deck is low in exile removal, but if my opponent was working off the assumption that I have 2x
Declaration in Stone then I think his choice was logical.
Ulamog is what I would choose. Basically, you have Declaration, Purge, Flare, Nahiri, Tragic (If he has other creatures out) and Bounce that can deal with it. Bounce will work on Ulamog, but generally, that is a desperation move. That leaves Nahiri, Purge and Declaration that will not hit Gaea. On the other hand, Outburst will kill Gaea, but be useless vs Ulamog. So, on pure card count, Gaea wins. BUT..here is how I would think about it.
At best, it is 6 cards vs 2 which would work in Gaea's favor. I am betting that you probably are not running Purge, or at least not 3 of them. so, I am guessing a likely 2-1 chance you have in hand. But, if you don't have in hand, you have a viable chump blocker I can see, and Gideon that will buy you all the time you need to draw an answer. Ulamog puts you on a short clock, probably dead after two swings from mill. I would place higher value on that.
I do see both side, and Revenge is more likely to live. But, until he draws an answer to Gideon, he has basically put the game in a stalemate until someone topdecks something, and I would guess you have more options generate draws (although he may have more answers to the stalemate then you?), not to mention stalemate tends to favor a mill deck. I prefer the more aggressive approach.