Oh, I forgot about the DreamWorld. Still, there are certain Pokemon like the Beasts and Unown which were unobtainable.Though Gen 4 has the asterisk of needing the event Regigigas to get everyone without transfer.
I think if we count events (Restricted giveaways, Mythics, dream world starters, Pikachu, etc), Gen 5 has everyone covered....except for Unown. Well. Had everyone covered. Dream World being down and all...
Speaking of the DreamWorld, isn't it annoying how soon it shut down? Only four years, and between the span of BW and BW2. The fact that it shut down so soon is infuriating especially since BW2 were very recent games. The shutting down of the Dream World basically killed a lot of potential ways to get with HAs like Lileep, Aerodactyl, and Burmy. The only way to get those Pokemon after the DreamWorld was shut down is to breed their descendants. Would it have killed them to keep the DreamWorld online for the 3DS era, or better why not make the 3DS game compatible with it?