I’m looking for the easiest and most flexible software that I can use to texture my 3d models made in Max.
Any suggestions and advice would be great, thanks. 
Artrage (about $60), Paint Tool Sai ($50), Gimp (Free), Photoshop Elements ($99). They are all flexible and they all work. Paint tool Sai has probably THE BEST pen/tablet input controls. Photoshop is used often, and is well known. ArtRage is niche but, it supports all formats you’d need.Gimp… been a while but I can’t imagine development has gone stagnant.
Although it is quite expensive, Mudbox is very tightly integrated with Max and lets you work on multiple texture channels at once very well.