Fairly crushing given the right POD and circumstances. Absolute best case scenario for Britain is obviously as follows:
1. New England secedes from the US
2. Establishment of Native British aligned buffer state in the West
3. Check of US expansionist urges
4. Control of the Great Lakes
Now personally I think the best case is if the British are able to dictate terms from Washington (which they would have been able to do OTL had it not been for what could almost literally be called an act of God) and if the British hadn't had to deal with Napoleon still in 1814-15.
Really the US got very lucky late in the war and if the Brits had been able to turn their full attention to the US you can bet that the US would have been crushed and Britain's full war aims established.
However, let's say a more effective British peace in 1814 still (Nappy being bested in 1813 let's say) Britain is effectively stepping on the US's jugular via crushing naval blockade, they manage to successfully occupy Washington, they have occupied Maine effectively and are cruising around the Great Lakes.
Ideally they would order the US off the lakes, followed by some territorial concessions in order to add some defensive depth to Upper and Lower Canada. That's all I can think of off the top of my head for this scenario