What products will help all depends on what's going on...I'm no pro, but here is my opinion:
If chaffing is happening, then something needs to either prevent the cause of the chaffing or limit the effects. So, talc will help absorb moisture (inc. sweat) which is a cause of chaffing, whilst baby oil or oily cream such as 'Bepanthen' (which I use) will lubricate the chaffing action, stopping it from causing friction damage and pain.
If it's skin irritation from sweat, pee etc that's causing a rash, again it's a case of preventing the cause and/or treating the symptoms. Barrier cream, such as 'Zinc and Castor Oil' will help form a layer between the skin and the pee, so reducing irritation. I think Sudocrem is similar, but I've never tried it.
If the skin is having an irritated or allergic-type reaction, antihistamine cream helps calm the irritation, cool the skin and cease the itching. Some nappy creams also contain vitamin D or something which helps the skin repair itself.
If the rash is infected, weeping etc, then some antiseptic cream might be a good idea. Some nappy creams also contain antiseptic.
Having said that, as we already know, there is no substitute for thorough cleaning, drying and letting the area get some air, but then again, the nature of using nappies points to the opposite!