'Best' is subjective and depends on your criteria. I'd start simple with visualizations...here's a few ideas:
1. Overlay plot run charts in Graph Builder.
2. Multivariate plot in the Multivariate platform.
Then if you want to get more elegant and analytic Fit Model. Make one series your response, the other the predictor and see how well one predicts the other? Or go straight to the Time Series platform and explore there by building various time series models and examine performance characteristics. Lots to play with.