i prefer 8x8 when you want a lot of stuff in the same screen and precision, but I like 16x16 too because it's easy to make sprites/tiles based on some games. As for 12x12 the problem is that then you can't work with xLIB tilemapper

I use 8x8.  But 16x16 is sometimes just as good an option.

I go for 8x8, not too much detail..I have no pixel art skills, so I stay away from 16x16

Well for me I always finding it more beneficial to use a multiple of 8. You can get some decent stuff with 8x8 but like you said the quality is low. Again stated alread 16x16 is more detailed but you wont be able to have a whole lot viewable on the screen but imho is your best option for quality and ease. So I guess it depends on what type of a tilemap you are creating.

Actually, 12x12 would be arguably only as slow as 16x16; you're still shifting 2 bytes around to draw the tiles.  The only difference is that you're ignoring the righthand 4 bits, a process that takes one AND command (1.3 microseconds)

But 12x12 sprites means more sprites to handle in the screen. It will be definitely slower than 16x16.

I have been thinking in exploring 10x10 sprites. Or, who knows, 8x10 or eventually 10x8.
Depending on what I do the remaining pixels could be filled with some border.

Is there any works of 10x10 sprites?

Have you seen the 12x12 with 16x16 sprites combination of upcoming Sonic game in UTI forums? Those graphics are awesome. They are worth some minutes looking at them.

I am doing some sprites work. I have some projects holding until I discover the sprites and I am stocking what I can find, rip and do.
I will eventually share the collection.

You can try your hand at writing your own lib...