That's a beautifully spiny form. Not sure if it's engelmannii or not, mostly due to pad size and shape. The spine formation and coverage of the pads looks like engelmannii, except the central spine formation seems more like phaeacantha to me.
The person who runs the opuntiads web site, Joe Shaw, usually replies to emails requesting an ID and he knows his stuff, so you might want to email him.
Re: Identifying Opuntias. It is challenging for me, unless I'm able to start with a fair degree of certainty of the major species group. humifusa, polyacantha, phaeacantha, engelmannii, fragilis. From there it can be easier if there's solid location data.
