There have been reports of him sighted in argentina, and we know that a sub full of high ranking nazis fled there. And we havent been able to identify the remains that are purported to be his.
So this is a very popular question, and while I'm sure there's more that can be said I'm going to link these brilliant posts from /u/Georgy_K_Zhukov. Spoiler alert; He's really dead.
Did Hitler really survive until the 90s?