For backend code, I'll almost certainly stick with Grails, but for the client side, I'm waffling between Angular and React.
Why Grails? It's what I know and like, and it works well. I can be productive with it, and I have something of an interest in promoting its use. I'm think that if I do some cool project using, say, React + Grails, that might turn into a cool talk for the local Groovy + Grails Meetup, etc.
Why Angular or React? Both are pretty modern, pretty popular, and claim to offer some real productivity boost compared to rolling everything by hand. Both seem to have plenty of traction and mindshare, and learning resources are available. I guess the same could be said of Ember and some others, but I kinda feel like the future right now is either React or Angular.
A third option would be to go with either Meteor or Node for the entire stack. And I am interested in learning both of those platforms, but not quite as willing to invest time and energy in them right this minute.