I had a lot of toys as a kid, and a lot of favorites, but since I'd rather not spend all day on here, I'm sticking to my 5 favorites - and a warning to you, there's a shitload of Transformers and MASK here.
Just look at that sexy car! This is the first MASK toy I owned besides that tiny Condor motorcycle everyone seemed to have. It was so full of awesome action features, I didn't even care that it turned into a boat instead of a jet - this thing was just way better looking that that weird gullwing-doored Thunderhawk.
4. Powermaster Optimus Prime
This is one of the biggest toys I owned and I loved that not only did it include a robot and a base, but that they could combine into an even bigger robot, and there was even a tiny little robot that unlocked the transformation. When playing with this guy, I'd often pretend it was a MASK toy as well by folding the cab over and imagining the smokestacks were laser turrets, not unlike Rhino (which I never owned, otherwise it would be on this list too).
It's funny - pretending Optimus was a MASK toy, and now I have a MASK toy I could pretend was Optimus. Only the base mode here was way cooler - a launching grapple hook and a giant missile made me forget the similarities with Optimus really fast. The fact that it came with two figures, including Miles Mayhem (even if he was wearing a different costume) was icing on the VENOM cake.
This was genuinely one of the most mind-blowing Transformers I ever owned. Sure, there was triple-changers that had two alt modes before, but this is the first one to have two robot modes. And they were different factions, allowing him to act as a spy on the Decepticons. The idea of spies would come up again withe the awesome Powermaster Doubledealer, but that was hindered by his lackluster bird mode. This figure however, had some of the best articulation of its time, and there was so much potential for fan modes - an airplane, a boat, a supercomputer - so it was often added to my MASK line up next to OP.
If Optimus above was one of the biggest toys I owned, than this was the one that topped it. I really loved the whole Micromaster concept - Matchbox car-scaled Transformers that had their own system of transforming bases - and Countdown was the biggest of the bunch. I had a few others, like its Decepticon counterpart, Skyhopper (which was much smaller), and the much enjoyed Erector. Putting them all together for a giant city was probably my favorite thing to do. No MASK connection needed.