Some of the knives recommended in this thread are absolutely ludicrous to consider having in the pocket of suit trousers. (EDIT: And I personally won't carry a knife in a suit jacket pocket for various reasons I won't go into.)
Anything over a 3" in blade length is off limits for me when I'm dressing up. But body size and wardrobe choice (even when limited to "dressy" attire) has a lot to do with how much stuff you can tolerate in your pockets.
If you're a big guy and/or you favor more traditional suit designs such as those sold at Sears, JC Penney, Men's Warehouse or any other mainstream American department store, then you might be more comfortable carrying a knife larger than those I've listed below. Traditional/mainstream apparel tends to be cut more generously for comfort, which allows larger pockets.
I'm tall, skinny and like fitted suits made of wool or something like it (not talking khakis or slacks here). In this kind of getup things tend to bulge and print easily and you look stupid with a lot of stuff in your pockets.
So besides light weight, I look for flat profiles in knives.
I go with the Victorinox Bantam Alox, which I continually recommend because I like it so much. It's very classy looking, besides being immanently practical and capable. And light.
Others I've considered are:
anything William Henry
Al Mar Hawk Ultralight
sub-3" Great Eastern Cutlery knife
sub-3" Mcusta of some sort
Moki lockback of some kind
CRK Mnandi
Fallkniven U2
Benchmade Shoki
Spyderco Dragonfly (pushing it because of its broad profile)
Fox Knives Elite
Benchmade 705 or 707
I don't have any of these, but plan to get a Moki Blakiston Fishowl lockback.