Personally I loath the Millenium bars. I've bought three separate batches over the years thinking maybe I just got a bad batch but all of them are the same. They're hard as a rock! Literally, like granite, nearly impossible to even bite through! And while each "flavor" has a different odor they all taste identical to me- like pure sugar. If you want to try them I'd suggest just trying a few bars at first.

Personally I really like the Datrex bars. Obviously when it comes to flavor and taste opinions will vary but the Datrex are quite tasty. They're a little crumbly but less sweet and kind of coconut-tasting. Kind of like a cookie. You can buy them in two sized of blocks and both have a five year shelf life if unopened, and once opened the individually wrapped sections are supposed to keep for a year.

There are some other single serving/400 calorie bars on the market that I haven't tried, too.

I'm not crazy about MREs. Some of them are pretty good. The beef stew tastes basically like canned stew, not terrible. All of them taste really salty to me but if you're sweating a lot the salt may be a bonus. As Montanero mentions you can save some space by removing a lot of the packaging.

Mountain House makes some really compact foods that I like. My faves are the LRPs (Long Range Patrol) packs. They're freeze dried but packed into tight blocks with the air removed, so unlike the stuff that you find at Wal-Mart or REI these are pretty small. The same basic stuff is sometimes found under the name "Pro-Pak" by MH. The downwide is that LRPs are sometimes hard to find and a bit spendy. But some of them are very tasty although again, a bit salty to my tastes.