There simply isn't "the best application" for mastering. And usually, mastering will take place in a well equipped studio (like for vinyl or cd mastering). Everything you want to do on your home or studio computer is premastering (even for mp3s if you plan to put them online).

A well produced track needs only a slight mastering, but you can also make a "not-so-good-sounding" track a killer. Usually, you want a compressor which has an analogue touch (I prefer T-Racks 24 by IK Multimedia), or another trick is to add a tube effect to your mixdown after it has been compressed. I usually leave a headroom of 1 or 2dB for future corrections. Remember that your mastering might sound very good on your own computer, but if you carry it to another place it might sound bad. So !always! listen to your track on many different audio systems (like on your computer with phones, with different kinds of speakers and even copied on tape on your crappy (or good) car audio). If the track sounds everywhere as you desired it, you are done.