How to Determine the Most Conserved Sequence from BLAST Result?
The "best" is the one with the highest ID score, lowest e-value and greatest length. There is no magic bullet, all of these are important. You should always look for the most similar sequence that matches as much as your query as possible with the lowest e-value possible. 

Furthermore, it will depend on what you are looking at. If, for example, you are blasting a protein with a specific domain and are looking to find its homolog in another species, it is often more important that the domain be conserved than that the whole sequence be very identical. If you find a sequence with high identity but where the domain is not conserved, that is probably not what you are looking for.