"I've never seen (something)" means "I have never seen (something) in my life" "I've ever seen" can not be the beginning of a sentence. It can describe something as the first or only time you saw it in your life. "This building is the biggest I've ever seen." Remember: "I've" = "I have".

I've never seen...

I've never seen - I have never seen (hiç görmedim) so...whenever you have this "never - asla" you know you will have to use some suffixes: mi, mı / mu, mü according to the famous harmony rules because it is a negative sentence I have ever seen - giderek gördüğüm --- here there are no such suffixes because it's a positive sentence, so it means you saw something for the first time or that you are talking about the only time you saw something in your life. For example: This is the prettiest girl I have ever seen. I want to kiss her now and I want to kiss her forever. --- Ben gördüğüm en güzel kız. Onu şimdi ve sonsuza kadar öpmek istiyorum. I have never met such a boring man. I don't want any more. --- BBu kadar sıkıcı bir adamla hiç tanışmadım. Artık onunla birlikte olmak istemiyorum. --- It means this man is too boring. You can't stand it, so you don't want to be with him ever again. I hope I was clear enough. Let me know if there's anything else I can help you. Iyi gunler.