AFAIK most game servers are in the same data centres as everything else, if they have good peering in general, they should have good peering for gaming.
No "good" ISP should be doing traffic prioritisation IMO as that is against net neutrality.
I recall Virgin did a deal to prioritise Playstation traffic at some point. But the thing is, you only need to do that if you are poorly managing your network so are over-contended to begin with, so I'd consider any ISP doing that the worst of the bunch - plus they can end that deal at any time. I have too many nightmares of how Plusnet slowly rolled out their traffic management all those years ago, which eventually caused me to leave them as it was throttling my traffic when it wasn't supposed to be. Even they ditched it eventually despite being the first to implement it.
This is what I like the most about Zen, they are trying to maintain a contention-free network (which obviously doesn't mean 1:1 bandwidth, just that they maintain enough spare capacity to cover peak usage) as much as possible. If you have no packet shaping but need your customers to be able to use VoIP or video conferencing, then you NEED to maintain a network with spare capacity at all times.