Most Usefull Research Path...

[Tifone]

I am so surprised Baalz hasn't entered the discussion to talk in favour of Blood yet :O

Anyway, I picked Blood - not only for his power but because of his *versatility*, which would be the most important thing IMHO if you can have only one path. Blood has everything from communions (aka Sabbaths), nice summons for every level of research (from the beginning to the late game with the infamous Heliophagi), nice (but maybe not exceptional) battlefield spells and great rituals - oh, and think about the globals: expecially if, as it has been done, you want to put 1vs1 paths in a game, Astral Corruption would be a 90% I-win-card no matter what for everybody not having the ability to dispel it.