Although Erza is amazing...she is not my waifu today. No it's someone that might just be more under-appreciated then Sakura Haruno (original waifu fyi).
It's Rin Tohsaka from Fate/Stay night unlimited blade works.
Rin is really awesome although extremely uptight most times... She is a true strategist and never gives up even when faced with the threat of death. She not only uses her brains but also her magic gandr shots and her mystic crystals. Ms. Tohsaka is truly a waifu to be reckoned with.