Best o-ring gasket material?

see below.

the same material can have different ratings - it depends on the exact stock. but they all have a rating. just look for the durometer number

- lower is softer.

an electrical junction box isn't a demanding application so epdm or buna-n should be more than adequate, the former if you need to protect against moisture, the latter oil. but if you have ozone evolution you probably want to revisit silicone.

as to repair, of course you can repair it, although the reason for original failure should be examined first. you can make a gasket cut to length - most of the industrial supply houses have o-ring kits with cord that you cut and glue. typically with loctite 404.

Viton's virtue is chemical resistance, which doesn't sound like something you need. It's pretty lousy on mechanical strength.

You've already ruled out silicone, which has the virtues of chemical resistance and excellent high temperature resistance, but is also lousy on mechanical strength.

Neoprene has excellent mechanical properties, which is why it is used for O-rings in demanding applications. Too bad that's not an option.

Buna-N has good solvent resistance, so it's good for fuel lines, etc. You don't need that.

EPDM has excellent environmental resistance and good mechanical properties. This is the one you want, if you can't get neoprene.

Any of these materials can be formulated across a range of hardnesses. You don't want to go too soft because that means the gasket will be more subject to cold flow (creep). I'd say you want the hardest material that would serve in the application, so that the seal is maintained over time. A soft material will relax and may lose the compression that keeps the joint sealed.

Note that there are different durometer scales. A Shore D 20 is about equal to a Shore A 70.