and influence buildings are mostly a joke aswell... ^^

I usually won't put an embassy on an Influence Tile unless there is a planet next to it that I want to flip (then when the planet becomes mine I'll put a Factory or Stock Market on that tile). Approval Tiles can be pretty handy once you start getting a planet's population up around 12b.

Influance is only a joke when you don't have a +40% racial ability   

The most useful building to build on a 100% Approval 'Mystic Spring' tile is the Counter-Espionage Center. It blocks spies off the planet it's built on and saves you those ridiculous nullifying costs, and gives you a 20% Morale bonus for free.

The best thing to build on a 100% Influence 'Ruins' tile is of course the Stock Market. It's real use is it's 25% Economic bonus, but it's extra 5% Influence bonus will get doubled anyway. The best things for spreading influence aren't embassies, but influence starbases and stock markets instead.

The Political Capital building also gives a huge bonus in both Influence and Morale, so building it on either of these tiles will generally make your Political Capital planet the happiest/most populated and/or most influential planet in your empire.

EDIT: If you have the Mind Control Center(which stops planet flips) and aren't going for the 75% influence victory, then the Ruins tile is worthless. Just build whatever you want on it.