Which ARM does best work to graduate to that has the economy of programming time, as well as the cost conveniences I have described
So far this thread is a perfect example of how everyone will try to convince that their particular favorite is The One. In reality there is a device which pretty much stands alone as cheapest and most popular ARM. A quick search on Ebay for "arm cortex board" (clicking on buy it now & lowest cost) will show what is virtually the only choice in the first few hundred listings. If you dont mind soldering, a small but perfectly functional development platform can be assembled in a minute or two for a dollar and change. For those who prefer to skip hardware DIY then the $7 board in those listings is ready to go. For mfg fanboys ST sells a compatible Discovery for about twice that. Because this is literally the "Most Common Arm Chip in the World" dozens of free toolchains are available. A fully functional ICE type debugger can be had for $5 from there too.
I'm sure someone will come along soon and challenge this assessment but mostly hacker types who insist on a new "chip of the day" each and every day. The more complicated and obscure the better. However overwhelming number of those listings tell the truth. I suggest Ebay, Aliexpress, TaoBao, and similar websites are an excellent weather vane for these kind of things.
BTW this is not my personal "favorite". ATM I'm satisfied with a completely different 8 bit platform and not particularly involved with ARM development. But did find it a bit thrilling to be able to program and flash an ARM device within 1 minute of download and debug/tracing few minutes later. YMMV (and probably will, often the best advice disappears in the noise).