A good choice would be something you actually like!
Otherwise, it can be a bit soul-destroying to work on a car and find that you haven’t really ‘bonded’ (for want of a better term) with it.
(Been there, done that).
Otherwise, a car that has a strong after-market and reproduction supply of parts will make your life easier.
Many classic US muscle cars are very well supported in that regard.
Otherwise, many German cars, especially (in my experience) VW (HUGE after-market spares sources, esp. for the Beetle) and BMW (Mobile tradition) are good for parts. BMW parts availability is epic, even when compared to mass-market cars like (say) Ford. Cost doesn’t have to be too alarming either!
Porsche is another, one mentioned in other answers. (Though cost could be an issue?).
Many British classics (Jaguar, MG, Triumph, etc.) are well-served for both reproduction, after-market and NOS parts. (Hell, people even make brand-new repro body-shells!).
A lot will depend where you live. Whilst parts for (say) a Ford Mustang might be easy to find in the US., shipping them overseas can be expensive…
However, I am assuming that searching-out parts for an obscure car isn’t what you’re seeking.
Some people enjoy hunting for rare, obscure parts for ancient vehicles no-one has heard of!