Like many others I have a number of soaps I rotate but when I'd be testing a new blade, I'd try to keep as many variables as fixed as possible (razor, brush, soap) and for the last year my go to soaps for testing new blades with were ones I was trying to finish; Proraso Green & Lea Cream, but now that I've finished them both and reading this thread I realized I no longer have a go to soap for testing new blades.
However in the 3 or 4 years I've been 'wet shaving' the soap I've consistently reached for the most is Palmolive Stick, and even though I really like using shaving sticks (have 4 on the go right now) it's clear to see the Palmolive one is the one which has gone down the most, but as I write this I'm remembering I really do like Vitos Red as well.