Considering you're on the straight six forum, you'll most likely be getting a biased opinion.

There are quite a few options for the later year trucks, but probably the most common engine is the 300 I6 due to it's size, reputation, availability, ease of service, and aftermarket.

I know I enjoy mine because it is unique. Of course, it'll never have the sound of a V8, but it sounds great and often turns heads simply because it doesn't sound like a V8. Everyone has a V8. Everyone and their brother has a V8. Not that there's anything wrong with them, but they're a dime a dozen in a truck. Whenever I bring my Bronco to a mechanic, I usually come back to find all of them huddled around the engine bay just to check it out because they want to see something new.

Seconly, with a couple mods and some work, they can be made to put out some pretty nice numbers. A 4bbl, intake, and a nice exhaust makes my 300 perform pretty nice, and I get way better gas mileage than most V8s do.

But, as said, it's all in what you want to do with the truck. Have fun!