Truth be told, the Ultra Series features a fantastic collection of monsters, and ranks up there with "Doctor Who" and the original "Outer Limits" for having such awesome creatures, both in designs and story fodder. Not to mention so many iconic characters that are as memorable as Godzilla's many foes and friends, including Red King, Gomora, King Joe, Baltan, the Capsule Monsters, and so many others.
However for me, here's my picks for the Ultra Series that had the most winners in the monster department.
"Ultra Seven" - Kind of a cheat here, but very good variety of bizarre aliens, done through traditional rubber suits, and even decent make-up effects on visible actors, with additional giant monsters thrown in for good measure. And to even such a point, that this series often featured multiple fantasy characters per episode. It's also the first entry in the franchise to really feature robotic monsters, something that "Ultra Q" and "Ultraman" virtually lacked (outside the Garamon androids of "Q").
And unlike "Ultraman", they didn't come off looking like they all were from the same designers and suit makers (which ironically, were the same exact people who worked on the previous series).
"Ultraman: Towards the Future" - the monsters here stand out for being truly unique characters, based on higher science fiction concepts that we really don't see in other Ultraman series, who are more often the standard monster stuff of the Showa-era past.
Gudis was a Lovecraftean manipulator of genetic cells; Plant Bios is a deceptive alien intelligence inhabiting a cybernetic plant body; Gerukadon a bizarre lizard / dinosaur hybrid more suited to "The Twilight Zone" more ambitious episodes; Daganja is directly inspired by lesser known Australian folklore and marsupials; and Majaba was a mutant locust of an exceptionally nasty origin, more akin to horror movies of the late 1980's than tokusatsu programs.
True, the suit execution on all of them was far from perfect, but character and concept wise, they fare surprisingly well.
"Ultraman Gaia" - despite the fan fare towards "Ultraman Tiga", I think the mistakes of that series in regards to monster designs (horribly short suits and suit actors, tile-style rock hides on almost all the monsters, and overuse of pastel / silver coloring), the testing ground was greatly improved by the time this series came along.
"Gaia" also thankfully abandoned the mostly two-dimensional 'jerk aliens' with a shallow superiority complex (from both "Tiga" and "Ultraman Dyna"), for full-blown monsters, ranging from primitive creatures, to mythical beasts, and Lovecraft-style alien overlords. And thankfully, there was a very good variety of designs, despite several monsters belonging to their own species and sub-groups (Liquid Metal Warriors, Weather Machines, Mezard Jellies, and so on).
Another thing I enjoyed most was that the monster suits were big again, over towering the Ultra heroes most of the time, and giving off a better impression as opponents to be taken seriously. Thankfully, they also played it smart and allowed some short scale monsters into the menagerie, like the oddly comical mutated heart muscle Mother Deents.
I have not seen "Ultraman Ace" as a full series, but from the monsters themselves, wonderfully crazy bunch, worthy of the name Super Beast, and again, big suits most of the time too!
Guess All of the Above Monsters' Names and Respective Places of Origin and Win a Prize!