What is the most easy to use/menu interface for this kind of converting tasks?

I personally like fre:ac: http://www.freac.org/

Fre:ac has already been mentioned, so I'll add QWinFF to the mix.
It's a frontend for FFmpeg so you can also use it for video files.

lame, of course.
Whatever fancy front-end you're using, it most likely uses this application in the background.

foobar2000, but you'll have to download lame encoder.

Online: http://music2mp3.com

Offline: http://www.mediahuman.com/audio-converter/
VLC can convert too but it's not so user friendly as you will have to go through too many options

Also you may go for flacsquisher

Lame with these parameters: -b 320 -m s -q 0 --noreplaygain

Just drag the files into a lame shortcut.

Foobar 2000 is good and versatile once you've set it up properly (get the lame encoder for MP3, I use it since the 90's ^o^)

I remember being quite fondly of Format Factory.

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Audacity does the job for me.

The best one I've ever tried is WAVMP3, but it's commercial, so you'll have to pay or *ahem* put on an eyepatch.

But if you're looking for freebies, there's free:ac and Freemake Audio Converter. I prefer the latter as free:ac is too slow for my taste.