Ok my favorite anime of all time only happens to be six episodes... it's called FLCL. It's a lot like the mecha anime Gurren Lagann which is also a must watch anime.
If you haven't seen Code Geass put that on the top of your list, it's a phenomenal (mecha) anime. If you'e looking for something a little more tragic, try Tokyo Magnitude 8.0. If you're looking for a light hearted anime try Eden of the East, the movies that follow the series are a must. Watch Fullmetal Alchemist Brotherhood if you haven't, a lot of people will consider it the best anime of all time and there is good reasoning behind that.
If you want my opinion on anything else, let me know 😃