I don't disagree with what has already been said. Here's my top 10 3rd party playability pieces/sets.
10. Not-reflector. Sure, he's small, but he's a LOT of fun. I'd love to see an up-scaled version. I have a pic of Unicron taking a picture of Primus using these guys. I wanted to make a photo comic, but I thought it might be a little risque for here. U: "Crystal city is causing glare. Can you just slip it off?" P: "That's what you said about Iacon. Pretty soon I won't have any cities on..."
9. FP Superion set. Are we counting add-ons? I'm counting add-ons. I love how Silverbolt looks with the combiner feet on his wings.
8. Herc. This should be 8 through 3, I guess... He looks good taking on Encore G1 Omega in my display. But I had a lot of fun as the pieces came in.
7. FP Insecticons. I love the Ninja idea. I don't know why. It is very neo for even my classics ideas.
6. FP Broadside. Mmmmm...Macross yumminess.
5. Hexatron. I have done all of the modes, and the hardest thing is that I think he looks so awesome each mode that I find it hard to switch to another. He spends so little in gun mode, but the rest are fun to switch between. I can almost do it now without referencing the instructions. Okay, not really...
4. Bovis. He. Is. Just. Awesome!
3. Topspin and Twintwist. These guys shocked me with their playability. Really, it's tough to pick 2, 3, and 4!
2. Hardbone. Lots and lots of fun. Plus, I can't help but be a 7th grader and snicker every time I say/read his name.
1. FP Code. I love this figure and have done tons with him. I have to put him away at times or I just continue to fiddle and fiddle. I need to go to work to support this plastic crack habit. (Quad U kinda goes here too, but Code is more fun!)
Honerable mentions, City Commander and Protector. They are at the back of my display right now and I haven't played with them in a while. But I liked to fiddle with them in the past. Also, MakeToys Battle Tanker is almost as good.
The future? I am so looking forward to finishing Feral Rex. And in a week and a half or so, I will finally get Chaos/Paladin in hand. Woot!