I like Mora knives and consider them a best buy and they are light for a fixed blade. You can get combination serrated/plain edged moras. I don't think much of serrated blades unless you are working with rope or other fibrous materials. A plain edge is much easier to touch up in the field. Moras are so inexpensive and effective that I would carry one as a first choice for solo hiking and inclusion in any survival kit. I get them at commercial fishing supply stores for $10-$12 each.
The Victorinox paring knives are cheap ($4.75) and super light– about 3/4oz. If you are only concerned with food prep and light repairs, cutting guy lines and so forth, they are a perfect hiking knife. Not much for true survival use though.
I like the Victorinox one-handed Trekker model for an all-around compact hiking and survival tool. The main blade does have some serrations, but they are shallow and can be sharpened easier than some of the narrow and deep serrations like the Spyderco knives have. The blade does lock and the one-hand model has a thicker blade than the other large locking (111mm) models.
The Victorinox Fieldmaster is my favorite for a non-locking model that has a saw and scissors.
Throwing weight and cost to the wind, the Fallkniven F1 is an excellent wilderness knife, but they aren't what I would consider ultralight gear.
Multi-tools are great too. If I could only have one tool or knife, the Leatherman Wave is my choice. As above, they aren't in the ultralight category. I use one every day for work, but I wouldn't carry them in the woods unless I was using a bike or other mechanical device that might need adjustment or repair.
Opinels are fine for food prep, but IMHO, that's about it. I find them hard to clean properly and the wood can swell. The carbon steel blades can be honed to excellent sharpness but they will rust without some basic care. I can undestand where they have appeal becuase of the wood handles and general design, but I consider them picnicking knives. I would choose a Swiss Army knife over an Opinel.
P.S. The Victorinox Farmer is possibly the best blend of size, weight and features, and a fine study in minimalist design. 3oz/85g and you can find them on eBay for around $20US. The Farmer has a large blade, saw, awl, screwdrivers, can and bottle openers. It lacks scissors to be perfect IMHO, but the simple addition of a Classic adds the scissors, a nail file, tweezers and toothpick, and builds in some survival redundancy with only a .7oz/20g weight gain.
