I'm currently using Foundation III and Intensive Therapy II. The numbers apply to the condition of your nails - I is for healthy nails, II is for soft peeling nails and III is for dry, brittle nails. There is also XTRA which is made for previously unmanageable nails.
Foundation III is a ridge-filling nail strengthener base coat that also helps to heal the natural nail. If you have any splits or peeling at the end of your nails, this will help to make the surface smoother for nail polish application. This is personally my favourite because as well as strengthening my nails it helps to keep my nail polish on longer.
Intensive Therapy II is for soft, peeling nails and helps to seal the layers of the nail together. Over a period of time it will even help to thicken the nail. It can be used as a base coat and a top coat but I prefer to use Foundation III as a base coat and Intensive Therapy II as a top coat.
Although many resources say that you shouldn't use any nail polish on your nails when they're damaged, I find nail strengtheners such as these two help to shield the nail from external aggravations.
I would love to hear if you have any tips on peeling nails, or if you've used any of Nail Tek's line before.