When you write it, you should use ‘네가’.
When you pronounce it you can use ‘니가’ as most Korean native speakers use ‘니가’ when they say it, but they write it as ‘네가’.
Some native speakers use ‘너가’, however, this is not encouraged by the academic perspective.
Why don't they pronounce as ‘네가’? Because it is confusing with ‘내가’(it means ‘I’).
네가 말했지. (writing O, speaking O)
니가 말했지. (writing X, speaking O)
너가 말했지. (writing X, speaking X)
References (in Korean):