The one Sum Sub linked is one I found comfortable and stays in, even when up and about. Same with this one:
http://www.lovehoney.co.uk/product.cfm?p=28124
Although I think mine is the large version of the above toy and I can't find it on LH. hmmm. Anyway, that one also was comfortable for me and my partner and it stayed in.
If you were hoping for something on the bigger, more filling side, i cannot recommend this one enough:
http://www.lovehoney.co.uk/product.cfm?p=9608
I recommend two things for a comfortable, long term wear plug. 1) Go for silicone and 2) Get one with a slim neck. The reasons are that the neck will bend and flex, meaning the plug moves within as you move (Unlike glass which is solid and won't flex with your movements and will feel more "there")