You can say both: To jest właśnie to, czego chciałem. To jest właśnie to, co chciałem. They both sound 100% natural. I personally don't feel any difference between them but I would use the first one when writing and the second one when speaking. More examples with "chcieć" Chcę psa. (psa- Genitive, Accusative- same form) 개를 원해 Nie chcę tego. (tego - Genitive- because of the negation (negation changes the case)) 이거를 안 원해