I guess for now it's where I am- Philly area, south Jersey (Gloucester county). I can get into center city in 15-20 minutes. I can get to PHL airport in 20.
It's affordable for a big city though I guess that's changing. I switched careers a few years ago and it is way less competitive for it here than in other cities out west, so I'm doing (for me) quite well.
It's really diverse here, this is important to me. Politically and culturally.
We have a very affordable house ($1k/mo including taxes) with a decent lawn and nice detached garage/covered patio that's paid off in the next year- live in one of a few little blue collar towns that's stayed under the radar for whatever reason. My husband said the taxes have been unchanged for years- they are lower than surrounding areas. I heard it's something to do with some industrial areas on the edge of town. Also (duh) no city tax here. I can walk 3 blocks to a pond stocked with trout with a decent running track around it and a tiny stream/woods that we see tons of turtles and occasionally turkeys and herons hanging out. We are in the same town as a good sized creek and can kayak either locally or all the way into the Delaware river. Weather is good, we can grow anything we want (I love gardening).
Fairmount park is one of the best parks/trail systems within a big city boundaries. Nice for foraging this time of year.
We can get to the Pine Barrens for chill hiking or kayaking (my husband's preferred outdoor activity) in ~45 minutes. Can get to the beach in a bit over an hour (if no traffic)
Great food of every thinkable style cuisine somewhere in the city or just outside (ok, MAYBE the Persian food is a little sparse, but there's at least one place and it's great). Good breweries/cideries/distilleries too (tho we've also homebrewed, have access to great/affordable supplies for this). The CSA I get produce/other local goods from is pretty cheap and great quality.
Most important thing for me is the diversity of music. Because Philly's been relatively cheap, a lot more risks can be taken here creatively compared to other cities. Great scenes in all the arts- theatre, dance, visual, music. In the last year my music career came back out of nowhere, I could afford my whole COL on it at this point. And I'm not playing soul sucking stuff- it's generally been a combo of a "Balkan, jazz, punk, and cheesesteak-inspired" band, a silly polka band, and a swing/jump blues band (with the odd free jazz, classical, Brazillian, salsa, or reggae gig thrown in). I seriously can't think of anywhere else one could do this especially at my level of playing/relative lack of networking skills.
My husband and I would eventually like to travel in the camper around the US and Canada and travel abroad but it's just really hard to think of another place whether here or outside the US with a similar music scene that would feel fulfilling. I tried checking out a few other cities in my late 20s-30s as well as a couple years abroad and it never worked out. I imagine we'll home base here until music isn't a thing I do anymore, and travel when needed. If I need a deep nature fix, I go on a thru hike.
If I give up on music as an income source then we may consider a move out West or to New England. But we aren't sure, and the reasons are basically being more in nature. It's hard to find a weather sweet spot though, and my husband and I have slightly different preferences. The mid-Atlantic is a good compromise. OR, we just keep moving to weather we like in the trailer. Another thought is my brother/his wife's whole family are in Pueblo CO and while prices have gone way up it's still relatively speaking affordable compared to other areas in CO, weather is not super extreme, plus we'd know people. My sister and mom are also considering that area one day, so we will see.
For old age, having a house not far from great medical care is a no-brainer. I'm in healthcare for the day job and have contacts both in Penn and Jefferson systems so feel good about options here. And again, close to an airport if stuff is unaffordable in the US.