This was asked before here. I'll give you short summaries for each Pokemon here though.
>The following Legendary Pokemon can be found in Black 2 and White 2. Detailed information coming.
>Main Legend
Kyurem - Giant Chasm
>Black / White Dragon Legends
Zekrom - Dragonspiral Tower (Black 2)
Reshiram - Dragonspiral Tower (White 2)
>Three Fighting Legends (Swords of Justice)
Cobalion - Route 13
Virizion - Route 11
Terrakion - Route 22
>Three Flying Legends
Landorus - Pokemon Dream Radar
Thundurus - Pokemon Dream Radar
Tornadus - Pokemon Dream Radar
If you don't know, Dream Radar is an app on the 3DS you can buy to get the Weather Trio in their special formes.
If you insert Diamond, Pearl, Platinum, SoulSilver, and Heartgold into the 3DS before starting Dream Radar (provided you have already caught Landorus), you will be able to catch Dialga, Palkia, Giratina, Lugia, and Ho-Oh, respectively.
>Legends From Past Generations in B2W2
Regirock - Clay Tunnel
Regice - Clay Tunnel
Registeel - Clay Tunnel
>Unlike previous games, you can catch all three Regis in one area. The difference is that you can only obtain Regirock at first, and you have to solve a puzzle which can only be discovered by visiting the area at day then at night. After obtaining Regirock, you will unlock the Steel Key in Black 2, and the Iceberg Key in White 2. You must activate the key via the Unova Link, and then you can obtain the respective Pokemon. You can obtain all 3 in one game if your friend shares the other key with you via Unova Link. All 3 are Lv. 65, further confirming that there is some villainous work going on keeping all these trio Pokemon at level 65.
Source: http://pokeds.com/guides/61
(the above comes from the answer I previously linked)
>Regigigas - Twist Mountain
Latios - Dreamyard (Black 2)
Latias - Dreamyard (White 2)
Uxie - Nacrene City
Mesprit - Celestial Tower
Azelf - Route 23
Cresselia - Marvelous Bridge
Heatran - Reversal Mountain
Most of these become available only after beating the Elite Four. Some of these are only obtainable in one version (noted above) Regice is only in White, Registeel is only in Black.
Sources: that question I mentioned earlier, IGN wiki page, Bulbabedia's DR page, source from the answer on the question I mentioned earlier with help from SneaselSnow, who commented on this answer
