So what you're saying is that you want the cheapest one you can get that will do the job.
Cheapest one? Look at your preferred distributor for their price-breaks.
That you can get? Here's what might be your biggest problem. There is a worldwide shortage of chips, and I suspect that some unscrupulous "distributors" are hoarding available stock to get the best price. Where we can, we've tried to get stock for what we suspect we will need for this year's sales, but it only makes the shortage worse for everyone else.
Will do the job? Your requirements are fairly easy to throw into a selection table. But things do get complicated. For example how many independent SPI, U(s)ART, I2C do you want? (SPI and I2C are much easier to share than UART). Do you want the UART handshaking lines (CTS, RTS)? It is only when you actually look at the individual device - either the data-sheet or with stm32cube that you realise that (say) one of the UARTs shares its pins with an I2C, so you need to drop that UART and use another. And only after building the prototype and writing the software will you actually *know* a particular microcontroller can do the job you want.
And let's have another look at "lowest cost". Even when you get into making tens of thousands of units, development costs (particularly software) are a significant chunk of the price per unit. So a slightly-more-powerful microcontroller where the code can easily fit might work out cheaper than one where you have to strain every last sinew to get things to fit into available RAM and FLASH. It will be on the market sooner, so you'll be picking up sales that otherwise go to your competitor. And watch out for your own marketing department insisting that extra features are needed during testing or just before release.
Arduino compatibility? That's something I'm not too sure about. Do you mean one where you can plug in arduino hats (or whatever they're called)? Will you write your own code for those hats, or need to use what's readily available on the internet? Even stm32 nucleo boards that have arduino connectors need you to write your own code.
The stm32L0 and L4 are good low-power platforms (L4 where you need more processing power). And the stm32G family are advertised for low-cost applications. Cost / power-efficiency compromise? Down to you.
Hope this helps guide your thought processes, even if it does not attempt to answer your question,
Danish
