If you are looking for "I missed you/ I've missed you" in a Romantic way, *also if you are going to see her/his(in case) face when you are telling this sentence* 会いたかったよ。（あいたかったよ。） or 会えなくてさみしかったよ。（あえなくてさみしかったよ） as selly san answerd would be good. Direct translation for these are "I really wanted to see you", "I was sad not seeing you." so this sentence doesn't work in text. Only works when you see their face in person or video calling. other two, 恋しかったよ（こいしかったよ） and 恋しく思っていたよ（こいしくおもっていたよ）sounds very cheesy and weird to me. It's typical way of non Japanese ppl trying to say I missed/I've missed you in Japanese. I can swear my Japanese guy friends never used this sentences. Also if you want to speak natural and casual Japanese, you better don't say "you" as 君（きみ）. People I can imagine using 君（きみ） for "you" are 1. Boss (men, not young) talking to other younger staff. 2. Prince. 3. Anime characters. 4. Foreigners. Also maybe you already know but in Japanese we often omit saying "you". If you want to say "you", then you can say their name. I missed you/ I've missed you in not a romantic way, like when I talk to my friend who I haven't seen/talked lately I would say "久しぶり！！"（ひさしぶり） which is actually long time no see in Japanese, but my friend knows that I missed her just by saying 久しぶり. Japanese people understand each other without saying things straight. In my opinion tho, I think "I miss you" is not common expression in Japanese. I'd say things like "I was thinking about you all day"(=because I missed you) instead. "I've been thinking about you" ○○(name) のことずっと考えてたよ。（考えてたよ/かんがえてたよ） is natural, and you can use it in text!

君に／〇〇(＝名前）に　（ずっと）　会いたかった（よ）／会えなくてさみしかった（よ） 君が／〇〇(＝名前）が　恋しかった（よ） 君のことを／〇〇(＝名前）のことを　恋しく思っていた（よ） 私が思いつくのは、これくらいです。 お役にたてばうれしいですが。