"Best" is going to depend on what your priorities are. I've been using Cyclemeter to log my routine training rides, and it's the best of several GPS apps I've tried for that purpose, but it's not really well-suited to touring.

I suspect that there's not going to be any one app that does everything you want, but that's OK. Apps are cheap. Some GPS apps have ancillary services like controlling music playback and shooting photos; with iOS4, if you have a newer phone, multitasking makes it less of an issue to have those features inside the app.

The one feature that would be desirable in a touring app would be map caching, so that you're not dependent on having a solid cellular connection. The only GPS apps I've looked at that can do this seem to be not so great in other ways—it's also worth knowing that Google's terms of use prohibit caching of their maps, so the apps that do this use Open Street Maps.

For route planning, the ideal would be to find an app that lets you import KML or GPX files, since you could upload other cyclists' routes, or Adventure Cycling routes, that sort of thing.

MotionX GPS does pretty much all of this, although in my experience it's awkward to use. I found its map-caching feature takes forever, and the whole design of the app is counterintuitive and overly complicated. But I'll give it credit for being a sort of Swiss Army Knife of GPS apps. But jack of all trades, master of none.

It hadn't occurred to me to use an app like TomTom because I would assume it would choose routings meant for cars, not bikes, and because I don't place a priority on having my app figure out the route for me.