tkk7406
sever91
Whilst I haven't been to Thailand I'm not sure if you'd find more foreign plates there (especially in Bangkok), I think amongst the southeast Asian countries Mayasia would be the best because of its location, but even there Singaporean plates are rare (unless you go to Johor), and Thai are even rarer.
Based on my observations:
Singapore registered vehicles are pretty common here than you may think, especially during festive holidays. It's easier to spot them along the E2 NSE (North-South Expressway). In fact, I can no longer count with two hands the number of Singapore cars in a small city like Seremban within a day. This is probably partly due to how cheap it is to spend here, as well as long-distance family across the region here.
Thai plates on the other hand are rarer, most Thai vehicles owners usually come here for work as Malaysia's salary is higher than Southern Thailand. Most of the Thai vehicles usually come from Southern Thailand like Songkhla, Yala, Pattani and Satun. Usually you'd spot them near Thai food restaurants, at least in my observation.