It depends a little on your wife’s chemistry and a little on the type of birth control you’re using. A water based lube is good if you are using condoms as a means of contraception. However, they have a tendency to dry out and many have glycerin in them which can cause irritation for some sensitive women. If you do choose a water based lube, you might want to find a glycerin-free version.
Silicone lube have become quite popular and work quite well.—-Its my preference anyway—They tend to last longer than other lubes, since they are not absorbed by the skin, and the possibility of an unpleasant reaction is minimal. However, not all of them are safe for latex, so it might not be a option if you use latex condoms. Also, clean-up can be a bit messier because they don’t absorb into skin.
Oil based lube is OK if you are on a hormonal method of birth control or not using anything and if there has been issues with water based or silicone based lube. It can increase breakage of condoms though, so I wouldn’t recommend using them with condoms. Again, clean up might be an issue since oil based lube would leave oil spots also on your sheets