Your going to hate this answer .....
It depends on what I feel like .... its instinct more than anything. I play Gigantic / Immense, and specialise worlds into dedicated Eco/Tech/Production worlds, each of the three categories being built on a ratio of around one specialisation for four of the other specialisations.
The early ones like microrepair(Thingy - whatever its called) and Xanthium Hulls, the one's I know that the AI is going to have a go at - then I'll use up a Tile on the Manu Cap world (I tend to build a Manu Cap world asap because of this) to make sure I beat the AI to it. I usually write off the Diplomatic one, I dont bother the AI is manic about this one, I let the AI get it, I play Terran so doesnt fuss me if the AI gets it - if they are building that, gives me more time to get the others.
Outside of those two, the other relatively urgent one is Harmony Crystals, and again I might use a Tile on Manu Cap world to do that, but usually try and think ahead and use a smaller world, starting fairly early in the game, so there are circa 18/25 turns to get it - ie start it early and forget about it, it'll turn up in the end usually before the AI.
From that point, because I play Gigantic or Immense, I've usually got enough production worlds on the go to dedicate one to each of the remaining smaller ones (Ultra. Friction, Eye, etc), as long as I get them going fairly early it matters not its not on a high PQ world. That then frees up the manu cap world for the real big ones and arguably important ones such as Restaurant of Eternity - the manu world is pushing that one out whilst smaller worlds have the others ticking over in the background. The AI will always go for Restaurant, so the quicker I get Xan and MicroRepair out the way, and start Restaurant the better.
By the time Restaurant is done, I have lots of other Production worlds lurking, so they sort out any others left, and the manu world cranks out ships. Not very structured
but it works for me
Regards
Zy