Anything old.
Bushka, Szam, old and collectable Brunswicks, Paradise, ect...
The cues that are extremely collectable now will continue to be collectable in the future and that $20,000 Bushka is only going to be more valuable in the future.
New cue makers and anything modern are a tough call and will likely never be as collectable as the old school stuff that came out when the idea of "cue collecting" was not really the point.
This type of question reminds me of comics. The most expensive comics you can buy came out when comics were not actually meant to be collected. Action Comics #1 with the first appearence of spiderman will always be the most expensive comic. New comics coming out as #1's trying to be collectable are a joke because half of the people buying them are taking them and throwing them into storage in mint condition and litterly millions of them are being stored. That kills the collectability when that many are being hoarded and kept in mint condition. The reason Action Comics #1 is worth so much money is not because it is old, it is because 99.99% of those comics were bought by kids who read them, threw then around their room, lost them, threw them away. Not alot of them survived and for one from back then before comic bags and boards even existed to still be in near mint condition is exceedingly rare.
Southwest cues as such will never be as collectable, they are not going to soar in price like a Bushka, because so many people are actually collecting them and EXPECTING them to be valuable in the future. There will be tons of Southwest cues 50 years from now still around, there will still be very few Balabushka's.