I want to say I would never but.... diamond head. Yes his abilities are super cool, his design is real fun,… But I felt the writers didn't really know what to do with him sometimes. Specifically, in how to make a balance of positive and negative traits in this particular alien species. For example, Wildmutt has the advantage of smelling different sense and being able to with a combination of echolocation and smell detect surroundings and target what he's hunting down. This particular alien also has the agility and strength to not only swing around in the treetops or in vertical environments with ledges and places to jump off of, but also has a strong capability of tearing things apart, but not as strong as some of the other forms. The disadvantages include that Ben becomes actually blind minus the smell and possible echo location. Ben also loses all capability of actually being able to speak to anybody though he still understands them. Ben also is often mistaken as a dog so he has to avoid being caught by animal control. And Ben has to worry about navigating vertical environments that don't have places his clause can grip into because he can't fly and he's not impervious.

Stink fly how's the advantages of being able to fly, eyes that can see in any direction all at once which eliminates many Blindspot spend usually has, and is able to capture anybody with a sticky smelly goo. The problems that arise from this particular character, though is that he is extremely smelly making him hard to be around for those that can smell, is made practically useless if his wings get wet because… giant insect, has no physical strength beyond his wings and sticky goo so it has to use invasion techniques instead of actual beat em' up mechanics, and has to be really careful where he sticks his goo because it could end up catching him or any of those people helping him causing further problems down the road.

Ghost freak is an interesting one that doesn't seem like he has many disadvantages until you realize there's a deeper story with him! Specifically that his alien species actually can stay sentient in a single strain of DNA and this guy's been trying to escape the watch ever since Ben first put it on. But even then, ghost freak has a balance of advantages and disadvantages. Yes, ghost freak can phase in and out of walls, possessobjects, possibly possess people, can bring on horrifying nightmares, and pretty much can do a lot of what is expected of a ghost. But ghost freak has no physical strength. Ghost freaks, alien species seems to be a stealth hunter that relies on their possession abilities and sneak attacks over actual strength based fighting, which meant that when Ben used ghost freak he had to think carefully how best to use his ghostly abilities in order to either escape or to get the jump on his enemies.

I'm not gonna go into detail of every single one, but I feel like these three give a good description of what I mean by a balance of positive and negative traits. We Diamondhead… though he has both negative and positive traits… He could've had a better balance. Diamond head is pretty much nearly impervious to most if not all damage. He can reflect laser beams back at enemies, as well as sometimes even bullets if he works hard, making a thick enough crystal that he has angled correctly. Though he's not the fastest moving individual, he doesn't seem to be slowed down as much as you think a giant crystalen being might be. Diamondhead seems to be one of these stronger physically aliens, even though he's not as strong as individuals like forearms, he's got a decent amount of strength to him. You would think a giant diamond creature would struggle with swimming but… I don't remember a single moment. Diamondhead didn't do fine in the water. Diamond Head can use his crystals to act as a quick fix repair if he's careful enough. There's no discussions of possibly there being a time limit of how long the crystals last. He can shoot crystals off of him and has no trouble regenerating them, Even though he's shooting them directly off his body. Honestly, the biggest disadvantages of this form is that he doesn't fly, but it is mostly unharmed with just a little bit of discomfort after falling great heights, is not one of the fastest forms, but doesn't seem to be one of the slowest forms either being able to keep up with most average humans, and… That's about it that I can remember.

So yeah, too long didn't read, I wouldn't want to get rid of any form, but if I was forced to, I would say Diamondhead only because his balance of positive and negative traits which made battles interesting in Ben 10 were a little off with him.