I went from Sparrow to Mailbox to Airmail. Sparrow was the best for its time. Mailbox had potential but was riddled with bugs. Airmail doesn't have the cleanest interface nor all the coolest features, but it's the best option I've found at the moment. For iOS I am still using Mailbox, but I guess now I have to use something else, will probably give the Gmail client a go.

One OSX client I've just found that I'm hoping does what I need: https://nylas.com/N1 It's open source.