I love pretty much all kinds of citrus type fruits. Oranges, Grapefruits etc...
My favorite ones are those "mikan" in Kumamoto prefecture (in Kyushu Japan). Cheap, abundant and just simply amazing all around.
Korean Jeju gyul is very good too. I like "han ra bong, but they are pretty expensive in U.S.
Since I can't get those far eastern ones easily around here... so I normally look something more locally grown, which are similar.
Satsuma Mandarins are very good - luckily I can easily get them in local asian groceries in season. They have stronger taste, but still very easy to peel and super yummy.
One of the delights of cold weather season. Love them. choke full of Vitamin C too.