Thinking about "what is my single favourite feature of Oracle", when all the products of Oracle (or at least all I know about) can be covered sounds stupid or impossible. There are far to many features which could be interesting, worth or made my life easier over my years in IT.
So I decided to pick something more generic.
The "feature" I like most is ONLINE.
With every release of the Oracle database, more activities can be done ONLINE.
Of course, customers want to do everything online, and many features, especially when they are new, require any kind of downtime, create degregations during their use or simply lock objects exclusive.
But after spending some years with different releases, read discussions on OTN and other media, speak about details and interna on conferencces, a kind of pattern is visible:
Even if right now something is not ONLINE doable, people think about possibilities how it could work. Or sometimes they write tools to circumvent the limitations or at least mitigate them.
It then often happened - in one next release, the feature can be used ONLINE, without the limitations anymore.