An oldie but a goodie - OPI Nail Envy in Original can not be beaten for conditioning and strengthening your nails as well as being a great base and top coat to protect your varnish. A true multi-tasker!
I always use this when I do my nails, or even on its own when my nails get quite brittle in winter. Hailed as a maximum strength product it really does work, and uses calcium and protein to naturally help nails look beautiful.
I know a lot of people who have used this after having acrylics for years and their natural nails are a mess. You can buy different varieties but I recommend Original all the way.