Depends on your standard diet...Biotest's bars are pretty damn good. Other than that, a high-quality trail mix is good (that is, a trail mix that isn't overdosed with too many chocolate bits, extra oils, and extra salt). Nuts in general are a good choice, you don't need too many to fill you up for the next 3 hours. Also, in some grocery stores you can get sealed foil-like packs of tuna and smoked salmon, they lie flat, they're small enough to generally be acceptable for carry-on luggages, and most of the brands don't mess it up with extra oils or multi-syllable mystery additives.
The main argument against that is, after you open the little metal envelope container, it might stink a bit and make you unpopular with whomever's sitting next to you. Add a couple of those mini condiment packs of mustard, relish, or (if your standard diet allows) mayonnaise and you're pretty much set. If it's not too late, also check if your flight offers a specialized dinner -- I've seen an airline offer Zone, low-fat, and low-carb meals if you request them in advance. If you can get that for one meal, the combination of maybe two high-quality MRP type bars and one or two medium (4 oz) sealed fish packets and you're set.