I am getting into writing songs and I wonder what key signatures you like best for guitar?

The keys that correspond to the first position cowboy chords are the most guitar friendly. That is E, A, G, C, D and their relative minors. Conversely Eb, which is a very popular key for horns and other instuments is maybe the least frienly key unless you tune down.

For maximum songwriting flexibility you can always use a capo to utilize less frienly keys.

I would suggest, if you are writing the song, try to put it in a key comfortable for you to sing in. Not much point in writng a song in , say, G, when your vocal comfort range is A to F.

+1 on what BlackAx says ... key it for your voice and don't worry about the guitar players. All keys have stuff that is inherently better sounding or playing and worse sounding or playing. All keys on guitar also possess their own groovy little "tricks" that can only be done in that key. And, re-tuning opens up 100's of other possibilities.