what is the best vlow bitrates aac encoder (8k,16k,24k)
im using comaact but is has been not updated for a long time
what encode should i use now, i need the hightest quality with the smallest file size,dont care about the speed

There have been no tests at those bitrates, but the new free Nero Digital Encoder is probably at least as good as anything else:

http://www.nero.com/nerodigital/eng/Nero_Digital_Audio.html

is there any GUI for it ?

Foobar 2000
Oh, and Nero 7 (the newest ones)...
Check out Doom9's audio encoding section--there's one being developed there, too.