I'll kick it off.. interested to hear what other folks around here think.
5K - Adidas Adios 3 Boost - just insanely fast and great for getting that quick turnover...
Half Marathon - Adidas Boston 5 - similar to adios, just with more cushioning
Full Marathon - Hoka Clayton 2 - best of both worlds.. super lightweight and stiff, but still some great cushioning
Trail / Ultra - Tie Hoka Challenger ATR 3 / Hoka Speedgoat 2 - both lightweight and built to go the distance in comfort - great in all distances up to 100 miles