What datatype to use when storing latitude and longitude data in SQL databases?

I would use a decimal with the proper precision for your data.

We use float, but any flavor of numeric with 6 decimal places should also work.

For latitudes use: Decimal(8,6), and longitudes use: Decimal(9,6)

If you're not used to precision and scale parameters, here's a format string visual:

Latitude and Longitude ##.###### and ###.######

To 6 decimal places should get you to around ~10cm of accuracy on a coordinate.