Yishun
Note that these may change over time. The HDB website will always specify whether an estate is considered mature or non-mature, for each sale exercise.
If you are willing to settle for a resale flat though, there’s no restriction on size and location. You may buy any kind of resale flat, in any location, under the SCCS.
Other requirements to remember:
You must not own another property at the time of application. If you just sold a private property, you must wait 30 months before you can apply for a BTO flat (this restriction doesn’t apply for resale flats).