If "memorable" means "easily remembered":
* Forgettable
* Unremarkable
* Bland
* Uninteresting
* Ephemeral
* Transient
If "memorable" means "significant" or "impressive":
* Trivial
* Insignificant
* Ordinary
* Mundane
* Unremarkable
If "memorable" means "evoking strong emotions":
* Uninspiring
* Unmoving
* Bland
* Unremarkable
* Boring
The best opposite word will depend on the specific meaning you want to convey.