What are some of the best flies for big bass and/or spawning bass?

Oh Lordy, that's like asking what's the best oil, or the best outboard, or pickup, or bass boat....... You tryin' to start a fight?

I'll share what I've learned, but with the caveat that I pretty much quit fresh water several years ago and mostly chase redfish now. Anyway, when I used to chase bass a lot I learned that day in and day out, you'll catch more big bass if you get your flies below the surface. I won't recommend any one fly over all others, but I used to have the most confidence in Clousers, rabbit strip leaches, and weighted crawfish patterns. Probably a black or purple leach (in 8' - 10' outside the grass line on a windblown hydrilla point) more than the others, but that's purely a personal opinion, with no empirical data to support it. But I think that where you throw it is more important than what you throw. That's my opinion, for what it's worth. And as folks who know me will tell you, my opinion and six bucks will get you a cup of coffee at any Starbucks.

Subsurface big hairy bass bugs & big streamers.. but know this.. you won't catch near as many fish with a big ol' scary fly.

You might check this out this podcast, fast forward till you get to the main interview. Can't say that I heard anything I disagreed with. You'll hear a lot of the same principles a conventional LMB expert would apply, but from a fly fishing perspective.

http://www.orvis.com/s/fly-fishing-guide-podcasts/4047

Secrets to Largemouth Bass Fly Fishing in Big Lakes - with Steve Hollensed
Published June 19, 2014
1:26:38
The main event is a long conversation with Steve Hollensed from Texas on his secrets to largemouth bass fly fishing in big lakes. Just to pique your interest, they are not always in deep water during the day, and fly selection is probably the least important decision you make.