Sumanai or Fergus

According to this table: http://fategrandorder.wikia.com/wiki/User_blog:Tamamo-no-Xeo/NP_Damage_Comparisons_(NA)#Saber
NP5 Fergus is leagues beyond NP1 Siegfried (22.7k vs 9.4k Damage).

Siegfried will get buffs at some point, so in the long run it might be useful to level him either way.

I got Sumanai as my tut servant.

3 months later and he's NOW at lvl 50. my aoe sabers are my friend supports.

yep.