I've pretty much loved all the post decade kaijin, with a few particular exceptions (A few of the dopants rub me the wrong way). Pre-Decade...it's very hit and miss. The Lords and Grongi were very awsome, but looked more challenging than threatening. The Mirror Monsters were kind of blain, but even they had some strong points (The partner monsters and Ray Dragoons.) The Orphenochs were all good, but I did miss colored monsters through that series. I know the point was to create the feeling of ash, statues, and just death, but it still made them pop less. The Undead were mostly badass (the Spider and Joker were personal favorites) and had a very creepy vibe to them. A few even played major parts in the plot such as the Peacock and Giraffe undead. The worms had a gross feeling to them, which was probably the point, and most had some kind of story behind their stolen forms. Most didn't have unique mannerisims in fight though and usually all did the same things aside from there unique powers. For a species mimicing humans they sure didn't learn alot from us about individuality. The Imajin were very fun both as heroes and villians. They all had personlity, interaction with human characters, and simple, but very nice designs. They even stuck to their own unique mannerisims and battle which means alot to me. The Fangire could have been alot more than what they ended up being. Some had nice, compelling stories, like the frog fangire and bear fangire, but most got reduced to MOTW status. The designs were nice, but only a few really stood out or appealed to me perfered aesthetic appetite.
The unique kaijin of Decade were neat, and lived up to the traditions of the worlds they called home. Apollo Gesit being the epicenter of these feeling due to just how awsome his suit is. The Dopants were all human, and that made them one of my favorite sets of kaijin ever. Each one had a back story and a very cool looking alternate form. My only complaint is that not all of them gained memories that suited their personalities or troubles. Even then, however, the suits were very unique and looked very good in action. I could write an entire rant about the Greeed and why they are so interesting, but I will keep it down for now. The Greed were incomplete for most of the series which was clear in their designs. When complete they looked sleak, powerful, and dangerous. Their incomplete states were very odd, but created that desire in viewers to see them at 100%. The Yummy were very neat as well, both solo versions and the chimera versions. While other series often fused man and beast smoothly, the Yummy were not pure combos. Many had human likes faces, while the chimera types almost looked like another species was bleeding out of their main bodies. It was fantastic! All this combined with the very human nature of the series, which was about life, desire, and doing what was right. The Zodiarts were amazing through and through. I don't remember a single one that I didn't like through Fourze. They were all themed after constilations, and represented them very well. The Horoscopes were also extremely cool, and you could tell the difference in power from normal Zodiarts just by looking at them. They weren't too gaudy or flashy, and they all had good back stories, even if a few felt rushed or under analyzed (I'm looking at you Lynx!). Finally we have the Phantoms who have impressed me so far. They are very simple in design, but still stand out from one another and have an organic feel to them. While a few look like they should be wearing some kind of belt, I think this creates a nice sense of living tissue as opposed to armor. That being said, a few of the CGI monsters seem a little forced...but over all look pretty neat to me. (At least they havn't pulled a Den-O and reuse the same three CGI monsters over and over again.)
I could go into the Showa era Kaijin...but that can wait for another day.