What bike?

I'm no expert and they're all good, but while all are pretty well supported for aftermarket / service items:

RM's are cool but they had a lot of small, oddball parts that were 1 or 2 year runs and then changed, and of course a lot is obsolete so finding parts can/will be a challenge if it is your main/only bike.

The same but to a lesser degree goes for the later Kawi.

Honda, a lot of parts have recently dropped out of availability as genuine parts.

If you look at magazine tests etc right back to the mid 2000's, they were already saying the YZ was the best overall of the Japanese 125's, so seeing as they still make them, the smart, logical choice would be a YZ out of the Japanese bikes.

The KTM thing, the ones I've had have been great ('19 tc250, '17 350sxf, '11 150sx) and I work on a few friends KTM/Husky bikes, no complaints. The KTM 125's rip engine wise, I'll say that.

Hope that helps and more folks can fill you in properly from here 👍

Honda, Kawasaki, and Suzuki if you want to be a little different and have some fun scouring eBay for NLA parts. It’s fun to do, but does get frustrating when one small part is holding up your whole motor build. 
 

YZ is fun to build, parts of all kinds are literally anywhere so you can do it on a budget and build yourself a nice bike. The KTM 125 had virtually the same motor from 1998-2015, so parts actually aren’t bad to find at all. If you know how to cross reference there are tons of parts that will work from all different generations/models of KTMs. 

Every 125 is flippin’ cool and they’d all be a blast to ride…BUT…strictly from a parts availability/easy to acquire and crazy good accessory support, I would go find a clean YZ and put your own spin into your own cool personal build and then go ride the crap out of it and enjoy it! 😎 👍❗️