Nexus did the best it could with the ending it was forced to have. So it gets nothing but kudos from me.
The original Ultraman series seems generally beloved, as does UltraSeven.
Return of Ultraman is a hit or miss affair, largely due to a lackluster final third.
Ultraman Ace I've heard mostly nothing but good things if you can get into it's goofy charms.
Ultraman Taro shares a lot of the same problems of Cosmos.
Ultraman Leo is generally well liked from what I've heard, though it's a strange duck of a show.
Ultraman 80 isn't talked about much, but apparently had some really cool monster suits.
Ultraman Powered is generally hated.
Ultraman Great is generally liked, though it has its flaws.
Ultraman Tiga is generally liked, but it's really slow.
Ultraman Dyna gets nothing but love.
Ultraman Gaia gets nothing but love.
Cosmos you know.
Nexus you know.
Max I haven't heard too many bad things about, beyond people not gelling with the set up.
Mebius you know.
UltraSeven X has a mixed reaction, from what I've heard.