Which Payment Processor should we use?
There likely isn't one clear winner here.

But authorize.net has been my go-to recently. That and PayPal Pro are built-in to Civi out of the box.

The Stripe payment processor extension is operating on 855 installs. iATS Payments is on 602 installs. 

Here is a very helpful review from Gingko Street Labs which also addresses the 4 aforementioned processors: https://ginkgostreet.com/content/evaluating-payment-processor-options.

I'm on WP as well.