Forneus

The demon Marquis Forneus is all about conversation. Forneus can make you an expert at rhetoric, which is basically just the art of talking to people and getting them to think and do what you want. Forneus can also teach you "Tongues," which I'm just going to go ahead and decide means "languages" in this context. If you've got any kind of plans to be an international playperson, you're going to need to speak the languages of the countries through which you plan on gallivanting. But the best part of Forneus? When you summon him, you get to take Spanish lessons from a giant sea-monster.

Foras

Foras is a president. When you summon this presumably democratically elected demonic official, he can help you out with all sorts of stuff, like learning "the Virtues of all Herbs and Precious Stones." The real reason to summon Foras, however, is because he can make you invisible. Just in the interest of full disclosure, some sources say Foras can make you invincible instead of invisible. Foras gets up in Andromalius' lane a little bit, as he can also find treasures and lost stuff, but any demonic summoner worth their ritual dagger knows the value of some planned redundancies.

Zagan

Summon up a demon who can provide you with a delicious beverage. Get at Zagan - both a King and a President, mind you - who will show up initially as a bull with gryphon's wings. My understanding of gryphons is such that gryphon wings are just eagle wings. The point is that he can turn wine into water, blood into wine and water into wine, which, by way of the transitive property also means that he can turn blood into water. 

Gäap

Gäap has the umlaut on his name. Gäap is really good for stealing a familiar from another magician (it doesn't come up a lot these days, but when it does, you'll be glad you know Gäap). But Gäap's real value comes from his ability to speedily carry people from "one Kingdom to another, at the Will and Pleasure of the Exorcist.".

Ose

Ose is another president, which starts to make me somewhat dubious of the demon world's political structure. But that's neither here nor there, because Ose is a demon whose skills you are definitely going to want to avail yourself of. At first, he's going to be a leopard, but just deal with it for a bit, because once he transforms into a human, you can get him to change someone else into whatever it is that you want.

Vepar

Sometimes you need something a little harsher. Sometimes you need Vepar. Like all the demons in the Goetia, Vepar is a man. But here's the thing: even though Vepar is a man, he shows up as a mermaid, which is kind of a real progressive move for a guy of his age if you really think about it. But sticking a thumbed claw in the eye of the patriarchy isn't the only great thing about Vepar. He can also kill people with putrefying wounds and sores, or even have worms breed inside of them.

Unless you've got something really weirdly specific going on, these are probably the best demons for you to summon. That said, I think I'd advise against it. I guess. Probably? Hell, what does it even matter. You know what? If you're gonna do it, just make sure you Periscope the whole thing. Deal? Deal.