There are various reasons for having a waifu, it might be due to attractiveness, behavior/character or just pure badass.
Anyway, most otaku's and weebs have waifu it's just what we do.
Here is the list of the top 10 best anime waifus
Erza scarlet
Erza, is what I would refer to as a model. She is extremely pretty and badass. She's strong, supportive, friendly and is Willing to do difficult things for her guild mates.
She is what I'll refer to as an epitome of beauty. She's not just the type to show it. She possesses a kind of "big sister personality" to those she cares for.
Mikasa