All and none.
No single unit should be enforced, both wallets and merchants must always have the option to easily convert between different units. For example when you want to purchase something worth $10 the merchant UI must show you the amount in bitcoin using the unit if your choice. For example you select bitcoin then it shows 0.00052500, you select satoshi it shows 52500 and so on. Same with your wallet.
BTC and m
BTC are 2 units I use and I feel they are enough for me.
Too many zero figures can distort the process I imagine the difference of value of the transaction I make and the fee I use for it. If my budget is small, I choose m
BTC. If my budget is not too small (0.01
BTC+) I choose
BTC.
satoshi explained about it that how he decided to give bitcoin a smallest denomination (in satoshi as 1 millionth bitcoin).
I remember this discussion, actually.
Finney, Satoshi, and I discussed how divisible a Bitcoin ought to be. Satoshi had already more or less decided on a 50-coin per block payout with halving every so often to add up to a 21M coin supply. Finney made the point that people should never need any currency division smaller than a US penny, and then somebody (I forget who) consulted some oracle somewhere like maybe Wikipedia and figured out what the entire world's M1 money supply at that time was.
We debated for a while about which measure of money Bitcoin most closely approximated; but M2, M3, and so on are all for debt-based currencies, so I agreed with Finney that M1 was probably the best measure.
21Million, times 10^8 subdivisions, meant that even if the whole word's money supply were replaced by the 21 million bitcoins the smallest unit (we weren't calling them Satoshis yet) would still be worth a bit less than a penny, so no matter what happened -- even if the entire economy of planet earth were measured in Bitcoin -- it would never inconvenience people by being too large a unit for convenience.