Thanks a bunch for all of the suggestions and input, everyone! I really appreciate it, and I had fun looking up all of these bots and watching their transformations.
I would recommend MP Ironhide if you want a good transformation. Not that he's super complex or time consuming, but the way the whole transformation comes together just felt so ingenius. I was very impressed with MP Ironhide.
I really liked this one, and it did look like a satisfying transformation. I'll keep it in mind for the future, but for right now I don't think it'd quite scratch my itch for something really involved.
Revenge of the Fallen Leader Optimus Prime is quite involved
Interesting and complicated, but I can't say I like the look of it at all. Really not a fan of the design of the robots in the movies. But this did meet my complicated requirement, so thank you for this.
Masterpiece Megatron aside, you'll definitely want third party. I'm guessing you're familiar with Xtransbots' Apollyon (Megatron). He had some QC issues when first released, but I've read the newer batches are better made, and in spite of the problems people have had with aspects of the toy's construction, there are a massive number of people who absolutely love it. The transformation is absurdly complicated; exactly what you're after.
The other figures that come to mind which you may not have checked out are the Maketoys Visualizers. This is a third party version of Reflector. Three bots, one camera. Complicated, well-made, and seems universally loved. Fanstoys have a version of Reflector too, but while the bots look more cartoon accurate, the consensus seems to be that the transformation is much more fun and interesting on the Visualizers. Both have a reputation for being a pain in the ass in terms of complexity.
I'll add more suggestions if I think of them, but those two are the mist compliated examples off the top of my head. Good luck, and let us know what you end up getting. One of the things that I really enjoy about TF toys as an adult is the transformation process, and while I prefer inventive and fun to complex for its own sake, I definitely appreciate the obsessive levels of engineering incorporated into many of these figures.
Apollyon definitely looks up my alley, but is it really, truly such an improvement over the gold version of the Masterpiece Megatron? I hear people say Apollyon is the definitive Megatron, and while the transformation looks really great, I have to say I really prefer the look of the robot and alt forms of the Masterpiece instead. I am prioritizing the depth of the transformation here, but since they both seem about on par from videos, I am inclined to give the Masterpiece the edge.
As for the Visualizers, they're definitely among the top of my list now. I really like the boxy look of Soundwave and this sort of has the same sort of aesthetic going on, and I like that it's three bots. Really like this one.
I'd really appreciate any more suggestions you can come up with, and thank you for these. Even seemingly obvious suggestions would be good since I'm still really new to Transformers.
If you can still get some for a reasonable price, Car Wars / RiD2000 were the pinnacle of complexity in transformations. I would in particular recommend Fire Convoy/Optimus and GodMagnus/Ultra Magnus.
https://www.youtube.com/watch?v=oNgWqrzVIoE
From the same series, Sideburn still holds the "most complex TF to transform relative to its size" award from me:
https://www.youtube.com/watch?v=tvXccs6-GRo
However, if you're willing to spend some more time, Takara's recent CW Devastator is simply a joy to combine and transform (and will take you quite a while), not to mention incredible-looking in all modes, especially combined:
https://www.youtube.com/watch?v=qx9l1b8STe8
Aside from these, Masterpieces and Binaltech usually have involved transformations and look really good (Binaltech might be hard to come by nowadays). I would personally advise agains MP Megatron because the only one I ever say broke a peg when his owner (one of the most delicately-handling persons you'll ever meet) was transforming it to show it to me, and frankly, it looks meh in both modes. Rather I would get the CW Megatron which is frankly perfect in every way unless you MUST have a Megs that turns into a gun.
Great, thank you for these as well! I enjoyed watching the transformations. I especially dig the very green aesthetic of the Devastator. A toy reviewer I like, Type V3, had some issues with the individual robots but enjoyed the overall combined mode. Are the individual bots fun? And I still need to check into Binaltech. Did your friend with the MP Megatron have the original print run or the gold version? From what I've heard the gold version fixed a bunch of stuff. Also, coincidentally, what brought me back to Transformers very recently was picking up the CW Gen 2 Megatron on a whim, since I had a similar toy many years ago. But now I really want a Gen 1 Megatron.
The MP line has really neat transformations, but not especially complex ones, although MP-10 (Optimus) and the MP-11 Seeker mold are satisfying. If you want really complex transformations, you're gonna want third party figures. My recommendations for complex third party transformations:
Badcube Wardog - MP sized Warpath, insanely complex panel "explosion" transformation.
Badcube Sunsurge - MP Sunstreaker, another "explosion" transformation.
FansProject Diesel - Motormaster. The ultimate partsformer. He's not really a transformer as much as a puzzle. Get the other four FansProject Stunticons for a nicely complex combiner team.
X-Transbots Apollyon - MP Megatron, super complex, probably the best Megatron ever made.
Commotus is a great figure but not especially complex. He does pretty much turn into a Metal Slug tank, though, which more than makes up for that.
Wardog and Apollyon were the standout figures from these. Really like Wardog's red paintjob and the transformation looked like what I'm looking for. As for Commotus, his resemblance to Metal Slug tanks is what really drew me towards him. He doesn't look especially complicated, but it does look like a fun transformation and I'll probably get him sooner or later.
Sorry for the long post!
Edit: One last thing. Is there any general consensus on the best Soundwave/Soundblaster, either first- or third-party? I really like his design and casetticons. MP Soundwave costs a bundle, but the Year of the Goat version is still $99. I also see a KFC Transistor which looks pretty good as well. Thanks again.