Been through quite a few, still looking for the best:
Disc Gator: Outstanding grip. In fact, the grip is too good when getting discs out of schule. Can get stuck on branches. Side note, the "pole" option they sell is pretty much worthless. Breaks apart too easily.
Disc Dog: Nice that it is lightweight and compact, but I have found that it does not grip as well as I had hoped.
Quic Stic: Very durable and compact. Nub on the end is ok. He is working on a different attachment. Depending on how that goes, this will probably end up being the main one I use.
Orange Ring Golf Ball Retriever: Great option. Used it for about 3 years. Downside is durability. I probably went through 4 of them in the 3 year period. The ring works surprisingly well on discs with no modifications needed.
RexArm: Heard good things about it. I have not used it yet, but have one on order.
If the Rex Arm does not work out like I hope, I will probably find a way to easily swap out between the nub on the Quic Stic with the Gator (Depending on disc retrieval situation) until the new attachment for the Quic Stic comes out.