What is the best birth sign for a knight build?

[Daddy Gibby]

Planning my first character out at the moment, haven't really decided what to do with him yet.

[cyäegha]

nine times out of ten, if you have to ask yourself that question, the thief is usually the answer
the lady is also usually a solid choice, as derived stat gains benefit from being boosted earlier rather than later

alternatively, while i wouldn't recommend it for a first playthrough, as it requires decent familiarity with the game, the atronach is good for a warrior that isn't planning on using much magic, as it gives you a massive defensive boon to enemy magic

really, your birthsign is not going to make-or-break a build like your skill assignments are

[Havan_IronOak]

I suggest that you go with the Lady for the early Endurance gain. That affects the number of hit points you get every time you level up so pushing that early is a BIG benefit. I "roll my own" class that I simply call "hero" which has the following major skills
Blade
Block
Alteration
Illusion
Mysticism
Acrobatics
Marksman
That gives you one skill of every school and allows you to more precisely control what gets boosted when. You might want to read this for some more complete info https://en.uesp.net/wiki/Oblivion:Efficient_Leveling