After having made several porchetta's, the best recipe (for the herb mixture, not cooking method) I have found was this recent one from La Cucina Italiana:
They use a pork shoulder with skin on. Skin on is really hard to find, at least in Minnesota. I also like to have porchetta that is sliceable, rather than using a shoulder which is more likely to fall apart. Nothing wrong with that, just personal preference. So I tend to use a pork loin rather than shoulder. The last one I made, I used a rib roast (6 bone?), which was excellent. A little difficult to cut table side, but having the bone in really gave the porchetta added flavor, and made for a pretty presentation. My roasting times and temp are different than the recipe because of the different cut of meat used - A blast at 400 for about 10 minutes, then go at around 325 for about 18 to 20 minutes a pound. I cook it to about 142 or 143F, then pull it out and rest it for about 15 minutes, by which time it's risen to about 150.
The Cucina Italiana recipe uses fennel pollen. It's hard to find, and expensive. But it definitely adds a layer of complexity to the finished product that fennel seeds alone can't add. After making a few porchetta's with the fennel pollen, I'll never leave it out. If you want heat, add a few teaspoons of red pepper flakes to the mix
Oh, and I usually add a tablespoon or 2 of olive oil to the herb mix. When using chopped herbs (rather than powdered, as when making a BBQ rub), I find a slurry containing a little olive oil sticks to the meat much better than using just the herb mixture with no added moisture..