You're greatest thermal efficiency will come if the panels are in parallel, since that arrangement reduces heat loss, as the panel temps rise.

Provided you use a header (large main pipe -- which your panels have) you'll ALSO get a reduced head loss (pressure loss) with the panels in parallel.

You'd get the highest out put temperature with a serial layout, but since your concern is to warm the pool, not provide hot water for a shower, that's of no benefit to you.