"Best" is not clearly defined, it depends on context so there's no way i can answer what best is.
In general you want to color manage, not doing so does you no favors. So unless you know what color management is, extremely well. Do not choose option discard embedded (don't color manage). It is reserved for very special situations.
Choosing between the two other options is a bit more tricky. In general i tend to favor keeping the original profile, if there is no obstacle why not doing so, as this does possibly destroy and change the data permanently. There are several reasons to convert tough.
- You are going to merge data from multiple sources and wish to convert to one of the profiles.
- You need to interact with unmanaged sources that play well in another space. Either because you have managed it that way or your familiar with results in that space better.
- Your end result will be converted to sRGB anyway. This is a bit dubious but might help if you copy paste screen captures for example.
- you nee dto do CMYK to RGB or vice versa..