In my opinion, most of the legendaries introduced in the fourth generation that really didn't play a huge part in the Sinnoh space and time legend were very forgettable. (In case you don't know which these are, the Pokémon that were part of the legend were: Arceus, Dialga, Palkia, Uxie, Azelf and Mespirit.)
Out of all of the (let's call them non-space and time) legendaries of Sinnoh, I would say that Shaymin and Manaphy are the most forgettable due to their physical and battling abilities being mostly lacklustre when compared to other legendaries.