Cerakote would probably be good. So would Roguard by Robar, and Black T would be awesome to.
To be honest, unless U want a silver color (hard chromed or some other similiar finish), just send the slide into Glock when it wears - It's only $45 for Glock to reblacken it - for any stock finish, the Glock finish is the strongest.
That being said, I just hard chromed a Walther p99 slide - it has tennifer on the slide like a Glock, but the black outter coating was not as durable.