Either I really don’t get this, or the place has changed a lot. I lived there roughly 2000-2005: loved it because it was fun and unique for the first couple of years, then gradually got bored silly. What especially drove me nuts was the lack of places to just … stroll. Again and again I was driving my scooter to Chenggong University to walk around there. Even though I lived walking distance from the campus, the sidewalks were so awful I drove my scooter to get there. Cycling … so many kilometers of busy flat polluted roads to get to interesting places. What I did love about Tainan was relatively easy access to long scooter drives up the South Cross-Island Highway, and camping at the top, but that’s not much of an option now.
Moved to Taipei (well, Danshui) and have never, ever regretted it. MRT (I am SO happy to almost never drive these days), much more variety of food (whether for restaurants or groceries), outdoor activities far, far easier here. Yes, more rain, but I think that’s balanced out by cleaner air. Admittedly Danshui does have better strolling opportunities than most other places in the Taipei area.
Oh yeah, moving from a well-furnished huge apartment with 10,000/month rent to a tiny shabbily furnished place with, I dunno, maybe it was 13,000/month, was also a bit of a shock.
Caveat: the money saved from several years living in a really cheap place were a big factor in later putting down the down payment on a mortgage in Taipei.
That being said: if you’re just arriving in the country, Tainan is a solid #1 choice. But living here long-term? Taipei first. Then … hm, a big gap to Kaohsiung (MRT, a touch more international), slight move down to Tainan at #3 (scootering everywhere, ugh), then Taichung #4. I know next to nothing about Taichung but have spent five or six weekends there over the years and never found enough to fill any of those weekends.
But a lot of my ranking comes from an intense dislike of driving a scooter, and a desire to spend a lot of time outside, whether walking or cycling or hiking.