i have 4800 ip finnaly and i want to buy a champ

well you just listed 3 completely different champions played in 3 different roles. What kind of play style do you want? personally I'd get Ahri since I really like her arcade skin and I main mid. But we need more to go on to give suggestions.

There are so many chanpions you can choose from, not just from 4800. Just pick a champion that looks fun based on your experiences facing them or playing them during free week. I'm not going to ask for your playstyle or what characters you already like. Someone once recommended Orianna to me because I liked control mages. Her playstyle didn't last more than a week before I gave up.

Always

"a cool champion that needs skill to master" is not the criteria for this. You should determine what role you feel like you want to play...ADC, support, mid, jungle, top. Lookup reviews for that role, which are best for beginners or most popular. "That needs skill to master" could land you a champion that's too difficult to get to enjoy. You can start out losing many matches and get discouraged. Anyway, the champion info on the champions on the LoL site rates how difficult they are from 1 to 10. You could look at that.

Then again, why don't you buy 10 champs at 450, well maybe 4-5, and try them out and go on from those you like?

Also try out the free champions of the week and you can ask people for opinions based on which you liked more.