Most Usefull Research Path...

[PyroStock]

Construction.

Cheap path boosters, SC items, per turn unit generators, assassins, supplies for large armies, underwater access for any army/unit, evocations, defensive items for mages, research boosters, gem producers, all the artifacts, communes, etc. No research path can claim to have it all, but Construction comes the closest.

In addition, if one could choose 2 research paths I would predict most would have Construction so they could enhance/access more of the other path more through items.

And what if unique items are grabbed by other nation choose const?

That could be said about almost any of the paths with their unique summons and globals. Even without artifacts you still have a wide variety of powerful items. The same cannot be said about Blood as without your uniques the best SC you could summon would be only an unequipped vampire lord or succubus.

If some MP game ever started based on limiting players to 1 path then the others might target the Construction nation just so they could hope to grab some items to make their uber mages or steal some nicely equipped SCs for themself. But with the right diplomacy the Construction player could make himself the invaluable goto guy for the needs of the others. And who wouldn't want to ally with the only nation who could make the good items? Besides you have to go kill that Enchantment Nation before they get Arcane Nexus up.