Comment by donohoe

Not sure about the best, but this seems to be the field depending on what your needs are...

  Active Campaign
  Boomtrain
  Campaign Monitor 
  Cordial
  Amazon SES 
  Eloqua
  Emma
  Experian Cheetahmail
  Exact Target
  Hubspot
  Maropost
  OneSignal
  PostUp
  Responsys
  Sailthru 
  SeconStreet
  Sendgrid
  SparkPost