That's a nearly impossible to answer question as there are so many different "good" answers that depend mostly greatly on your desire.
Pure carnuba wax is well known for the extremely deep, lustrous shine it can produce, but it's not very long lasting and takes considerable time and effort to apply with the finest quality carnuba extremely expensive.
Synthetic spray-on/wipe-off products are at the opposite end, giving a reasonable shine, relatively long life and very easy application.
My personal preference is for a high quality liquid polymer similar to "Deep Gloss Paint Sealant" by Wolfgang. It gives a very good shine, has an exceptionally long life and is (to me) a bit easier to apply than carnuba. A good coat of carnuba over such a product will give an incredible shine that while sort-lived does not affect the underlying (much longer lived) polymer.
No "wax" alone however will do much for an old, neglected finish. If you want to get a beautiful look on an older finish, I would highly suggest that you begin with rubbing clay and lubricant. Such is time and effort-intensive, but is EXTREMELY rewarding and is the closest thing I've discovered to a true miracle product.
Myles, Spend a while surfing around in www.autogeek.net. There are several discussions on the best combinations of carnauba waxes and synthetic sealants. Ever seen an 8 oz. cup of carnauba wax for $150 ? They have it......and everything else on the planet for your paint.