uh... I use "Any Video Converter" on Windows. Ok for me ley, I think is the settings when you choose the format that determines the quality.
Free software usually takes more time to convert than paid ones I guess.. Like e.g. I shoot @ 24fps, I will convert back to 24fps, same resolution(1920x1080), then format maybe you need e.g. avi(codec also can choose i think but I not sure how this works so I don't touch), then audio AAC/mp3, bitrate 320kbps(or match ur camera settings), that should be it. Usually if you choose formats like .3gp = super loss in quality but that's because of the format.
I'm not an expert but just sharing and hope it works for u!!
PS: If you use Mac, there's this app on the Mac app store called "SmartConverter" but its very limited and takes quite a long time to convert. And.. (not suggesting you do this) there are many "free" Pro versions of "Any Video Converter" lurking around. If you can find, then good! If not, the free one should do the trick~
Google mediacoder. Its extremely powerful, multicore supported, and loss of quality is not a problem if you know how to use it properly. Only issue is that there are many options and parameters to set, due to how powerful it is.
Handbrake is usually the first choice for easy video processing.
If that doesn't get you what you want then you can try AppGeeker Video Converter, heaps of different formats to choose from..
These are all easy to use.