I'm pretty sensitive to smoke as well. I've also stayed at about eight different Las Vegas properties in the past two months. Here are my rankings for least to most:
Mandarin Oriental
Four Seasons
Aria
Encore
THEHotel
Wynn
Bellagio (really bad, even in a renovated lakeview room on a non-smoking floor)
I think in general I like Aria best- high ceilings, well ventilated, easy to get outside quickly with minimal exposure to the casino, available gambling floor when you want it.