It's going to be a bit tricky if your funds are low, since a lot of the more complex TFs tend to be larger, older, and/or more expensive than not.

MP Megatron is apparently considered pretty tricky, but will be costly to acquire and you may have to jump through a lot of hoops to get him depending on your states gun laws.

DotM Leader Sentinel Prime is another apparently-complex one. You might be able to find a loose one for cheap somewhere.
I gather RotF Leader-class Optimus Primes was pretty complex too.
Most of the on-screen characters from the Bay movies were pretty complex IMO. Leader Brawl is my personal most-difficult-to-transform TF, but I'm not a big movieverse fan.

Beast Wars Transmetal 2 Megatron is often said to be a bit tricky, though he's not usually listed with the likes of the above.

A lot of the Maximals from Beast Wars Neo are apparently pretty challenging.

IIRC the Binaltech/Alternators series was fairly complex by TF standards.

If you can find any of the Robots in Disguise/Car Robots Autobot brothers (Sideburn, X-Brawn, Prowl), they're meant to be pretty tricky. They'll probably be some of your more affordable options. Sky-Bite might be worth a look-see too.

Beast Hunters Deluxe Smokescreen and/or Prowl both have a certain degree of challenge to their transformations.


A few more cost-accessible TFs with transformations that are at least interesting IMO are: RotF Voyager Bludgeon (and it's a great piece too, though it really does need a better head and swords), Hunt for the Decepticons Voyager Highbrow and Deluxe Terradive, and Dark of the Moon Scout Ransack. I'm sure other people can name a few more.