I honestly can't say what the most distant object is I've ever viewed. I'd have to look through my logs. With DSO's, you really can't push a large amount of magnification. The incoming light is dim enough that it would be squashed by it. Some galaxies are just at the edge of your vision, some are so large, you can't get the entire thing in the field of view - the Andromeda galaxy is like that.
The Orion nebula is another spectacular sight that won't fit into the FOV. It extends like wings. The Ring nebula is an awesome sight - a smoke ring suspended in space.
Planet's are different. On a night with great seeing conditions, I can push up the magnification and see some good detail. Saturn is always smile inducing. On a good night, I can see the Cassinni division between the rings and cloud detail on it's surface. I can make out different land masses on Mars and Jupiter's cloud belts and Great Red Spot.
The Moon - awesome on a clear night. I can literally look into craters.
There is just so much to see up there, and I have my favorites that I revisit.
Now I'm jonesing to take the scope out, but it's cloudy here.........