Generally, if a letter-writer surpasses a floor of having recognizable credentials (Ph.D. at good institution, or professor at good institution), it's better to go for the more familiar professor rather than more famous professor.
With that said, in your case it may be worthwhile to pick #3 (along with #1-#2) because he's at the U.S., and may have a more extensive network there. And as for your worry, deadlines for letters are not that important; at virtually every grad program there are a few weeks between application deadlines and the start of the review process, so you can send him/her a reminder if the letter is not submitted on time.
You've been an RA for #1 and #2 so I'm not sure if an additional letter about a brief RA project from #5-#6 will add much value. And #3 seems to dominate #4 as an option.