I like any double jointed snaffle, egg butt or loose ring are my favorites. The double joint helps the bit lay flatter in the mouth, providing a clearer signal to the horse and it's more gentle on the mouth. I like french link or bean mouth middle links best.
A bit with copper or any mixed alloy metal will help softening the mouth, like sweet iron does. I have all SS bits right now, but have used bits with a copper roller center or copper inlays with good success in the past. It just depends on what your horse likes!
I am not a fan of rubber or plastic bits. They can be thick, which bothers some horses, and they can also be drying to the mouth. Some horses really like them though, so again, it just depends on the horse!