Tru-Peep "Super Hunter" sized peep. Simple and very light as far as peeps go.
No problem on my Liberty at 30" , in fact I am actually using this size to shoot 3D and it works great. I can just see around the edges of my Spot-Hogg's white ring for perfect center alignment.

On a peep related note.......actually string related. That peep of mine hasn't turned, rotated even the tiniest bit since I have the bow and I assume by now the string/cable are settled completely. I have seen no string creep at all. Peep is straight back everytime.....very solid, very simple.