Best format for recording time stamp and GPS
As Spacedman put it, "best" is pretty subjective. However, as we have found, a good format for time is Unix time (aka POSIX time, aka Epoch time). Most databases support it and it is still pretty human readable.

For location, we like decimal degrees as it is easy to read and stored and is compatible with Google Maps API. It's also easy to convert to other formats if needed.