사랑해 - Sa-Rang-Hae is the best. No Korean people speak '저는 당신을 사랑해요.' and '나는 너를 사랑해.' It's so formal.

저는 당신을 사랑해요. （a bit more humble) ---> Jeo-neun dang-sshi-neul ssa-rang-hae-yo 나는 너를 사랑해(요) (more casual) ---> Na-neun neo-reul ssa-rang-hae-(yo) 사랑해 (most intimate, but subjects have to be clear) ---> ssa-rang-hae [Romanizations are more like pronunciations. NOT official romanizations]

Sa-Rang-Hae