I'm not sure if ipe is a great choice for this, but I think it deserves an honorable mention because I've never seen insects eating it. The trouble with it would be needing to predrill holes (because it's so hard), and it's a bit more expensive than oak.
Regarding insect control, use borates. That's the only think that will really work out in the long run. I wouldn't trust cedar to be strong enough for a sill plate, but even if it was, I've seen bugs eating it (cedar shingles and siding). Yeah it's better than pine for resisting bugs (for a while), but bugs will attack any soft and chewy wood... and cedar will eventually lose it's protective oils. Treated lumber uses ACQ (Alkaline Copper Quaternary), but they had to abandon the use of arsenic because of tightened regualtions. Treated lumber just isn't as protected as it used to be... hence the growing popularity of alternative decking materials (like ipe and composite).
Oak is a good choice (tried and true). Ipe is as hard as oak knots.