The result you should choose depends on the context of your analysis. The "xcorr" function represents "the similarity between a vector x and shifted (lagged) copies of a vector y as a function of the lag." If your goal is to find the exact shift needed to align the signals, you should select the highest correlation value, whether it occurs at a positive or negative lag.
Since you mentioned that the spike at lag 0 is the largest, this indicates the signals are best aligned without any shift. Therefore, if you're looking for the best overall alignment, you can use the lag at 0.
Refer the below documentation on "xcorr" for more details on how the function works:
Hope this helps!
