If you don't mind the theoretical disadvantages of a heavier, wider tyre, Metz 260 or Avon 250 (about the same width). You should probably switch to a matching front at the same time - at least I would.
I really like my Pirelli Night Dragons, but most people seem to buy the Metz and Avons. Anything is better than stockers. I don't see a big difference side by side with a 250 and 260 versus the pirelli 240. I have parked them side by side mounted and it is hard to see a noticeable difference. Pirelli, Metzler, and Avon are all good reputable tire companies.
It seems to be everyone has a thought on what is the best. I have rode one with a 260 metz, and now mine with the 250 avons either way they both seem better than the stockers. Just choose what you like. Price may matter, it did for me I wasn't sure metz or avon. Although with all the discounts and the rebate avon had going I couldn't pass it up.