Definitely 1846313750000000!
1846313750000000 was the first sat of the block in which it was mined, which makes it uncommon. (Sats after the first are all common.) You can see this in the decimal representation, which is 644102.0. The decimal representation is BLOCK.OFFSET, so anything with a 0 as the second number is uncommon or better.
My strat so far has been to offer bounties for interesting sats. Bounty 3 is still outstanding, but it is very hard.
(1846313750000000 is currently sitting on an Opendime on my desk.)