
I left out Zearth, Nice, and Dark Fantasy and just kept full series starring an Ultraman. Narrows it down a bit...
I have to pick Mebius (or Moebius, which sounds cooler but apparently isn't official) for its great amount of fun but also its great mix of that and some darker story elements from time to time. Plus it has Ultraman Hikari, the Ultra Brothers, and Semi-Frequent appearances by UltraFather! Top it off with a great cast of characters and some awesome monsters with lots of personality (see: Reflect Seijin), and you've got yourself an awesome show!
Behind that is Max, followed by Nexus.
So my list'd be like this:
Mebius
Max (FUN!)
Nexus (Great story)
Gaia (Pretty nifty show, feels pretty epic)
Tiga/Dyna (Haven't seen enough of Dyna, but Tiga's got some really great moments mixed with some really dull ones. Also, lots of it feels kind of dated. But it has Rena, so I'm not complaining.)
...
Cosmos (Not as bad as some people say but I can't STAND the characters. Omg.)