When I started I told myself I'd only get the Sith and maybe some Empire characters, so I passed on all the "core" ones. Now I've extended it to generally Force Users. Mostly because most of my favourites are from the EU and they'll never get figures, so it's better to have a Luke than just have Dooku represent SW for me. And also because while the average Sith is cooler than the average Jedi, I’ve gotten a bit tired of just how 2-D they all are, so I’m just going by which Force User has the coolest lightsaber, robes and feats. Any “ideology” that is to be found in both Orders is downright laughable. Anyway, for me, I'd consider the Jedi & Sith across the Eras the core of the collection, so I'd focus on them (which I am technically doing). However, if you were to break it down by Era, you'd have non-Force Users as well, such as Han & Chewie, Rex from TCW, Tarkin, and so on. For most people the core of SW is:
>Luke (ANH, Bespin, ROTJ)
>Han
>Chewie
>Leia
>Ben Kenobi
>Vader
>Boba Fett
>Emperor
>Yoda
>C-3PO + R2-D2
+
>Anakin (ROTS)
>Obi-Wan (ROTS)
>Ahsoka (TCW)
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I think most would stop there when they're talking about the "Core of SW". Then you have fan favourites and characters with expanded arcs who became a big part of the Canon later, such as Maul, Grievous, Lando, Rex and so on. The ST is such a mess that beyond Driver being a great actor and making Kylo cool, I don't think anyone would call them essential. Maybe Old Hermit Luke because the look is great and you can pretend it's some EU Luke or something. Now with the Mandalorian we've got more characters coming in out, with Din himself becoming a fast fan-favourite. But I doubt anyone would call Bo-Katan a core character.
As it stands, I've never really felt the need to complete certain casts because I've just never been enough of a Han/Chewie/Yoda/etc fan to throw the cash at them, much less now when they're going for aftermarket prices. However, the difference between them and something like, say, the Avengers, is that while I don't have the slightest urge to buy a complete Avengers team (I’ve always loved Iron Man a lot, but I never fully got into the Avengers or the JL or whatever that much; with Capes you have to focus on only a select few characters) or get all the figures of the new fad/hit of the month, if I had infinite money and space, I'd go back and surely buy all the SW stuff and make small dioramas about key scenes from the Saga. But, as it stands, I have set limits so I'm focusing only on the few characters I like. Beyond that, I don't much care about having them be from the same movie/show and just buy whichever costume/look I prefer.
Personally, my current plan is to get Obi-Wan & Anakin from TCW, a Jedi Master Luke, wait for the inevitable ROTJ Vader, cross my fingers for a TROS Sidious and a ROTS Palpatine and we'll see where I'll go from there. But if I had to pick the smallest circle of essential SW characters, I'd go with:
>RotJ Luke, Emperor, Vader
>ROTS Anakin, Obi-Wan
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You get the core characters of both Eras and then you build around them if you wish to continue. You can go the Empire’s way and stock up on Troopers and Moffs, you can build the Jedi or Sith Orders and so on. You just have to juggle it all with all the other licenses you’re interested in.
