Assuming we're talking about single player, as Castile I'd take the Big Three after Exploration and Expansion. The Big Three are Admin (for CCR and GC), Diplomatic (for more and better diplomats to prevent coalitions from ever forming and for lower warscore costs), and Influence (to cheaply integrate the many vassals I will create and then absorb over the course of the game). Religious is also useful for Spain--more and stronger missionaries are always nice as a major Catholic colonizer--although since you won't benefit from Deus Vult if you pick it sixth, an argument can be made for grabbing it sooner. Trade is a good pick too. What you don't see on my list anywhere are military ideas. I never take them because they're completely unnecessary once you learn EU4's combat mechanics and how to grow efficiently.
Here are some screenshots from my 1.33 Ironman Castile run in 1578. First, North America:
Steam Community: Europa Universalis IV.
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Every time a tribe or federation attacked one of my colonial nations, I Enforced Peace and grabbed another 15 or so provinces. The net result is that I'm ridiculously powerful in NA.
In case you think I didn't do anything but fight tribes, here are two shots of Europe. I'm Emperor, have smashed the Protestant Reformation, and recently revoked the privilege. My collection point is the English Channel, which will become even more lucrative when I finish eating England and take the rest of Low Country provinces that are part of it.
Steam Community: Europa Universalis IV.
steamcommunity.com
Steam Community: Europa Universalis IV.
steamcommunity.com
South America and Africa have also gone well:
Steam Community: Europa Universalis IV.
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Steam Community: Europa Universalis IV.
steamcommunity.com
The ideas I picked were the ones I listed in the first paragraph (although of course I haven't worked my way through the entire list in 1578).