1. Basically Everything in Mobile Fighter G Gundam
We really couldn't do this list if we didn't at least address the stroke-inducing insanity that was G Gundam. Honestly, we could just carve off the five most ridiculous Gundam designs from that one series and call it a day. That wouldn't be very interesting, though, so we'll just get it out of the way right now.
Despite the series' popularity both in Japan and the U.S., many take issue with G Gundam even now. Specifically with its departure from the operatic cosmic conflicts that defined the original Mobile Suit Gundam universe. G Gundam was instead more like a martial arts film writ large -- about 60-feet tall and made out of metal, to be specific.
As in most Gundam series, the story begins with most of the planet Earth packing its bags and colonizing space. Despite (literally) being self-contained, self-sustaining bubbles in the vast infinity of the void, these neo-nations aren't too comfortable around each other. Rather than let war ensue, they agree to hold a "Gundam fight" every four years to elect the winner as president of space.
G Gundam wasn't just a departure from the then-waning Mobile Suit Gundam brand thematically. The machines' designs also took a bizarre twist. Each country (save, of course, for Neo Japan) incorporated a different racist stereotype into their core design. These ranged from the simply offensive -- like Neo Mexico's bandito-inspired Tequila Gundam -- to the offensive and insipid -- like Neo Holland's Nether Gundam, which was (literally) just a windmill with limbs.
The title of most absolutely bonkers design from G Gundam, however, goes to Neo Australia's entrant: Jumping Gundam. Not just because it's (literally) a six-story kangaroo with metal boxing gloves. Not even because it carries another, smaller Gundam inside of its massive pouch. But because Jumping Gundam is actually piloted by an actual, live kangaroo.
You see, in the G Gundam universe each mecha is controlled by a skin-tight suit that monitors the pilot's movement. We also see this control method applied to another animal in the form of series antagonist Master Asia's horse, Fuunsaiki. That's slightly less ridiculous than Jumping Gundam, if only because the horse was used for travel, and not to determine the fate of an entire galaxy.
2. Big Zam (Mobile Suit Gundam)
The primary Gundam universe, a.k.a. the Universal Century, is a breeding ground for absurd mecha designs. Being the longest-running and most prolific of all the timelines in the Gundam multi-verse, each new creator must be tempted to find something the fans haven't seen before. Given that the first series, Mobile Suit Gundam, aired all the way back in the 70s that can't be an easy task.
Big Zam has no right to use that excuse. Airing in the 36th episode of the series overall, this walking fortress was admittedly a late entrant in the first season. Still, we have to think that someone -- whether they were show's fictional weapons designers, or the actual artists -- could have thought up something more practical than a green potato with chicken legs.
Despite being susceptible to such high strategy as, say, a swift kick to the knee, Big Zam was actually pretty effective in its time. Clocking in at around three times the size of an average Gundam, this monster was fitted with weapons that were simply too big for normal mecha to wield. This included the particle cannon at the center of its body which could destroy several space borne battleships in a single shot. That must have kept the opposing forces from laughing too hard. Well, at least for a bit.
You see, another major flaw in Big Zam's design was power consumption. So much energy was used to operate the thing that it could only fight for up to 20 minutes at a time. That's a pretty serious defect for a target as tall and impractical as the Leaning Tower of Pisa.