Whatever PoW algorithm you use, an attacker can set himself up to solve it optimally. Legitimate users are stuck with mobile phones, smart credit cards, or Javascript in browsers with limiting PoW capability.
I disagree, and I outlined a solution
here. You guys didn't think hard enough.
When I say proof of work, I don't mean proof of work to validate the transaction I mean proof of work to create the XRPs. Users with mobile phones, smart credit cards, etc.. would have to go out and buy their XRPs on the open market. Which is the end-game for Ripple anyway once all the XRP are given out.
Eventually, the requirement that
Ripple users fund their new accounts through XRP purchase on the open market is the ONLY WAY for XRP to serve in the anti-spam role.