Stay away from Sparkpost. They will put your account on hold without any warning (and for no good reason) and then their support team takes ~2 days to reply.
We use mailgun but there are a bunch of wierdly dropped emails. Am I to hope that would change with something like Postmark if not with Sparkpost? The fact is that we always use the lowest tier of paid plan since our volume is not too big and dont feel that we are an "important" user on a lot of services.