I guess criteria would be: guaranteed to be open, and decent conditions.
Lower 48 USA.
On a good year Arapahoe Basin will have a 60-80in base in early May and be close to 100% open. Its high altitude and mostly north aspects preserves the snow quite well and can be winter like even in early May but the SW facing Zuma bowl can be super fun and spring like from March.
BUT, this year conditions are extremely bad. Snow just does not want to fall.
They can still have a monster March and April so be vigilant of the weather there in the next few months and decide then.
If I had to bet I would pick a Resort in California...
It is awful hard to gurantee open and good conditons.
Snowbird is usually open seven days a week in early May, but I think last year they were down to three day weekends only and closed mid-May.
Another lower 48 possibility is Mt. Backelor. I think they are havin a good snow year so far.
When looking to book a May trip, you really do not need to worry about booking this far out. No matter what resort you will go to, it will be late season and accomodations cheap and plentiful.
I would wait until later in the season to decide, and wait to see how the snowfall shapes up in Jan, Feb, March. Look what happened last winter for example of why now its best to play the waiting game.
The usual suspects are: