What do you want to take pictures of?
For my Peru trip last year I was totally convinced I was going to bring my 100-400... ended up not being able to fit it in the backpack with everything else, and traded it out for the 55-250. While the 100-400 would have been better for bird photos, those weren't the main point of the trip, I was still able to get "good enough" photos to remember some of the cooler birds we saw, and the space savings were huge.
15-85 is a pretty good range for most things. If you want to capture more of a scene than 15mm will fit in, you can (almost always) shoot a pano and stitch later. Doesn't work well for the ultrawide perspective, but does work will for broad landscapes. Depending on your reason for thinking you wanted the 10-18, this will either be points in favor of bringing it (if you want that foreground exaggeration) or leaving it (if you just want to fit the whole scene).
The 85, to me, is primarily a portrait lens. Hardly ever comes on big trips, because I want to include more background in any portraits, since the point is usually to capture the memory of being there. Ditto the flash- unless you plan on taking a lot of outdoor portraits and kicking in some fill, I don't really see the point.
So that just leaves the 55-250 vs 100-400. I have a bit of a soft spot for the 55-250, but there's no doubt the 100-400 is superior in a lot of ways. It just boils down to whether that superiority is worth the extra size and weight... and without knowing anything about what photos you want to take, we can't help you there.