Collectibility is absolutely in the eye of the beholder. Some collectors are of the "everything in the <xyz> line" type. Some are the "one of each family" type.
For old Macs, some don't care at all about the LC line, others ONLY care about the LC line.
There are a few easy guidelines, though.
1. Was it only produced in small numbers/for a few specific markets/etc?
This includes "unique" systems like the Macintosh TV, the not-in-the-US PowerBook 550c and PowerBook 2400c, and the limited-production-run Twentieth Anniversary Mac.
2. Is it particularly "historic" in the genesis of the Macintosh line?
Obviously any "first" system falls in here - the original Macintosh, the Macintosh II, the Macintosh Portable, the first Power Macs, etc. But this could also include the
design historic systems. The original iMac, the flat panel iMac G4, the titanium PowerBook G4, even the Cube.
3. Does the specific one you are looking at have some specific provenance?
Things like developer machines, the "clear case" late-stage prototypes, machines that are verifiably owned by someone of importance (
Douglas Adams' Mac IIfx, for example.)