Based on the average of recent polls:
Clinton and Sanders would beat Trump by 9 to 17 points respectively. Clinton would tie Cruz, while Sanders would beat Cruz by 11 points. Clinton would lose to Kasich by 8 points while Sanders would be in a tie. In short, Sanders is the strongest Democratic candidate.
Trump leads Cruz by 11. His closest lead was 10.
Clinton leads Sanders by 9 points nationally, closest yet