[Ninjinister]

With classic Turtles, some of the weird randos like Hot Spot, Halfcourt, Sandstorm, and especially Scratch are sought after. idk about the others but it was stated that the mold for Scratch literally broke.

When it comes to Next Mutation, Turtleflage Leo's Camo Blitz Cycle and Turtleflage Don's Camo Hydro Skimmer are two toys you'll hear a lot about reaching higher figures. Scratch is usually the first name people think of for "rare TMNT figures" but even just the vehicle or figure alone loose out of these two usually sells at higher than a MoC Scratch.

2003, the only really "rare" one is Quarry. There are others that are less common though like Bishop, Karai, Blanque, that era.

Hard to say about anything made after that because after the internet became a more common thing for most people, you started seeing people scalp right out the door. Muckman from the 2012 line was considered hard to come by until Playmates started including them with a Pizza Points box tier. I personally never saw the 80s line from the Wanted: Bebop & Rocksteady line (2012 Raph and Leo dressed in trenchcoats, Bebop and Rocksteady with color schemes like their 80s versions).

Everything after Rise's first wave was hard to find in some areas and I never saw anything but restocks of wave 1 over and over. Nothing really hits big prices though that I'm aware of. Shredder might be the rarest but I'm not sure.

I also personally have never seen anything but the four basic Turtles in the Mutant Mayhem line either in stores. People seem to be finding the secondary characters in abundance elsewhere though, including wave 2, and I think Splinter is the only one that was changing hands for $50+ for a while, but that may have died down when more figs got to market. This is another one where I've seen pegs get emptied only to be replaced with the same stock over and over.

Of course depending on your country, there might be harder to find ones than others, and I know UK has had exclusive figs and variants over the decades.