I just started getting schematics, but not really sure which ones are more useful.
More end? Crit? Def?
useful in what sense? To sell, or use them on your toon?
If you mean to sell, I'd go with Fortitude, Overkill, and Critical since most good healers, tanks, and smashers stack those.
If you mean to use, it just depends on your role. I'm sure people can comment more if you specify your question