Oh yeah, definitely plenty to choose from 
Iāve always been a fan of some of the weirder aliens: Wildmutt, Stinkfly, Ripjaws and so on.
Eye Guy is also freaky cool, and was quite obscure before Omniverse so I always liked him too.
Hell, I even liked Arctiguana and Ditto because Iām a weirdo like that
Probably the only two aliens in the original series I donāt like as much were Benvicktor (or Frankenstrike, depending on your preferences) and Spitter. Benvicktor was fine, of course, he had a cool design.
But it was the first alien that didnāt really have a unique powerset, and the tech elements in alien design kind of started here (the megawatts are more of a weird case because they were apparently generated from a quasar - did I tell you how much I love this series yet? ). He was strong and such, and also had electricity powers.
Ben hadnāt unlocked him at this time, but Buzzshock had already been introduced as one of Ben 10,000ās aliens at this time and did all this and more as indicated by the Megawatt episode:
I donāt hate Benvicktor, I like his aesthetic despite the weirdness, and he only ever appeared in one episode of the original series anyway. However, he did sort of set some of the trends that began the franchiseās decline as a whole, imo.
And then thereās Spitter.
Spitter, like Arctiguana and a few other future-related designs, was actually taken from one of the concept designs for Upchuck before they decided on the final product:
You may recognise a couple of these other designs as the future villains āSplootā and āMot Snikrepā (Tom Perkins backwards, the name of the designer of this batch of goodness, who seemingly took over after Dave left the project fairly early on - respect).
Anyway, Spitter was only ever seen in use by people who had access to master control and could instantly switch forms at will, meaning they were usually only Spitter for a few moments at a time, and used his one discernible ability: spit, to chain into other attacks with other aliens. Because of this, we donāt really know his full capabilities, but if this is the only trick he has, it makes him pretty useless on his own merits.
Moving on to later seriesā¦
I kinda liked the whole winged and cloaked thing Big Chill had going on, even if it was difficult to replicate in toy form
And Echo Echo, despite having powers already previously in use which are easily abused in the form weāre presented, has a really neat name and a fairly solid design to match (Echo, sound, obviously, but also one that is a copy of an original source, which is repeated to show that copy too). The sound design (fittingly) is also the best for him out of pretty much any alien from his era. His sound effects are as iconic and memorable to me as those from the original series.
Brainstorm (great name by the way) isā¦ a strange one. As far as I saw when I was younger and could only see a few of the episodes, Brainstorm was basically smart, and could shoot electricity from his brain, and that was it. That was what the name indicated, what profiles about him I read stated, what he could do in the Ben 10 game creator (remember that?), and what the episodes I happened to see when I was just catching what I could on TV showed.
Taking it all in as a whole, itās much more complicated.
Sometimes, apparently, he has telekinetic abilities, either by zapping stuff with his brain lightning or whatever, or even making himself fly by proxy. It baffles me that I completely missed this when I was younger to the point where I wonder if they just sort of threw this stuff in occasionally and just never mentioned it to anyone. Very odd.
If he does actually have telekinetic abilities, great, butā¦ wouldāve been nice to see it some more, ey?
Anyway, then there was Spidermonkey (again, these names are a treasure):
A very cool, weird, unique concept, even if I feel the execution couldāve been better in his design. This is still an admirable attempt however, you can clearly see both animals in him, even if one is more pronounced.
Humungousaur, as discussed, missed potential. Bit more tapped into when he grows, but even still, eh.
The others I donāt have much opinion on one way or another.
Lodestar was a cool powerset with a really weak design. Rath was funny.
Already discussed the Andromeda 5, definitely discussed the ripoff era.
Okay, Omniverse.
In the alien department it had, without a doubt, a lot of garbage.
Atomix, Iāve discussed. Astrodactyl is just what? Where does the jetpack come from? Whampireā¦ what about Whampire. He was a waste of a decent concept, the only thing he has going for him is the mind control spittles. The four joke aliens were given actual powers and it was dumb, moving on. Ball Weevil meh, Bullfrag meh, Bloxx meh, Kickinā Hawk was so meh that apparently whoever made this picture forgot to include him. Gutrot I like the idea behind, gas powers are sorta cool, but eh.
The real big boys are Feedback, Crashhopper, and Gravattack. These were the three aliens that gave me the hope that they knew what they were doing now. It was misplaced, sure, but Iām still thankful for these at least.
Feedback was yet another electric-themed alien, but given his design was by Dave Johnson he had a really solid design that reflected that of the original series.
Crashhopper has a power that doesnāt seem very entertaining on paper: jumping around, but is very fun to watch
And Gravattack, who has easily one of the coolest powers of any of Ben 10ās canon aliens ever: gravity manipulation, given that heās a living planet. Itās to the point where Iāve considered replacing my current favourite with Gravattack. Heās that cool.
However, Iām afraid Iām going to have to go very traditional, and stick with my old favourite: XLR8.
Heās one of the first aliens I ever set eyes on, back before I even understood the premise of the show, from some stupid little ball with his image printed on it. When I eventually started watching the show, I was already attached to another blue hero with access to insane speeds, you might know him.
So it was no contest really. Plus, heās a friction-manipulating raptor with a retracting faceplate - whatās not to love?
Apologies for taking so long to reply by the way, I got a bit caught up