[Saulot]

Assuming these are serious questions...

With Dominions 3, you'll soon discover there is no 'best' for anything really. Though there are some more popular strategies.

--What D3 nation is most powerful in summoning rituals ?

This question is a red herring. Summoning what? Each nation has magic paths that it specializes in, and this naturally means completely seperate summon groups.

Take EA Agartha and LA Ermor for example; the latter is set up to summon tons of undead of all different kinds (though a lich/ghost king does help significantly) while the former can spit out Umbrals, and with Umbrals, you really don't need any other summon in the entire spellbook (though some would argue that everyone needs a Tartarian Factory).

So is one better than the other? No, just different.

--What would be the best pretender desing to suit well with summoning ?

Well, that should be obvious, right? One which has the magic paths you want for summoning what you want. Though some can be better just by adding increasing their paths. For example, if you want a Ghost Factory, the difference between a Lich with D4, and one with D9 + Skull Staff is quite big.

--And what would be the best research path choice for summoning ?

Once again, depends on what you want. Though generally the Conjuration school has the most, followed by the Enchantment school. Just look at the spell list in your manual.

In the future, it would help to be a lot more specific with your questions. These sort of generalized/'the best' questions will most likely bring blank stares from most Dominions players.