It completely depends on where you're based and where you want to go...

LHR-HKG strikes me as a good choice as it's relatively cheap, just squeezing in under 6k miles, plus HKG-LHR has no carrier charges if booked as a one-way (so you book two one-ways) and Cathay Pacific has very low surcharges on LHR-HKG as well. But it isn't so much a special offer as a generally good used.

Travel within Asia and Australia is another excellent use of avios.