Best L series lens for Astronomy Shooting

I thought most people shoot crop sensor for telescope astro as they usually dont provide an image circle big enough to cover ff sensors.

With the type of astrophotography you seem to be describing, you'd really need a mototized mount and would be much better served with something like a 90mm refracting telescope with a good field-flattener. Then you can shoot Andromeda galaxy and the nebulae in Orion, for instance. Cost-wise it still wouldn't run as much as a large, fast Canon L lens. A bigger, longer lens only exacerbates the main problem, which is that sky objects are in constant motion.

For the moon, you will want a sharp telephoto. Unfortunately you will usually be limited by seeing conditions if you shoot at a long focal length. You don't need tracking gear for the moon, only a sturdy tripod.

An APO refractor would be best, but in terms of Canon lenses, you could get a used:

Canon 400mm f/5.6L
Canon 300mm f/4L

These lenses would be nice for DSO imaging as well, if you get an EQ mount.

For stars, it depends how wide you want. Samyang 14mm is popular for wide field allowing you to have a nice foreground and sharp stars. Mainly you need to be weary of coma on the edges.

Any sharpness you get from your lens can go down the toilet if you don't have a proper mount. Honestly if you are only mounting 5lbs, and stacking short exposures at these short focal lengths you don't need anything too crazy. Hell, if you are shooting star trails, you don't need a proper EQ mount.