Event exclusive Pokemon: Mew, Celebi, Jirachi, Deoxys, Phione, Manaphy, Darkrai, Shaymin, Arceus, Victini, Keldeo, Meloetta, Genesect
Evolution families that aren't on the regional Pokedex and you can't obtain using only one copy of B2: Bulbasaur, Charmander, Squirtle, Caterpie, Pidgey, Ekans, Pikachu, Oddish, Paras, Diglett, Machop, Bellsprout, Tentacool, Geodude, Ponyta, Gastly, Voltorb, Exeggcute, Cubone, Hitmonlee, Kangaskhan, Mr. Mime, Scyther, Jynx, Tauros, Porygon, Articuno, Zapdos, Moltres, Mewtwo, Chikorita, Cyndaquil, Totodile, Ledyba, Slowking, Misdreavus, Unown, Slugma, Houndour, Stantler, Smeargle, Miltank, Raikou, Entei, Suicune, Ho-Oh, Lugia, Treecko, Torchic, Mudkip, Poochyena, Zigzagoon, Wurmple, Nincada, Whismur, Minun, Torkoal, Duskull, Chimecho, Snorunt, Gorebyss, Regice, Latias, Kyogre, Groudon, Rayquaza, Turtwig, Chimchar, Piplup, Starly, Kricketot, Shinx, Burmy, Cherubi, Glameow, Snover, Rhyperior, Dialga, Palkia, Regigigas, Giratina
source
source
Pokemon in the previous 2 lists that aren't used by any trainers: Bulbasaur, Ivysaur, Venusaur, Charmander, Charmeleon, Charizard, Squirtle, Wartortle, Blastoise, Caterpie, Metapod, Pidgeotto, Pidgeot, Ekans, Parasect, Bellsprout, Ponyta, Haunter, Exeggutor, Mewtwo, Chikorita, Bayleef, Meganium, Cyndaquil, Quilava, Typhlosion, Totodile, Croconaw, Feraligatr, Ledyba, Pichu, Misdreavus, Unown, Porygon2, Lugia, Ho-Oh, Treecko, Grovyle, Sceptile, Torchic, Combusken, Blaziken, Mudkip, Marshtomp, Swampert, Wurmple, Nincada, Shedinja, Whismur, Loudred, Duskull, Chimecho, Snorunt, Regice, Kyogre, Groudon, Rayquaza, Turtwig, Grotle, Chimchar, Monferno, Piplup, Prinplup, Empoleon, Staravia, Kricketot, Shinx, Luxio, Snover, Dialga, Palkia, Regigigas, Giratina
source: reading pages like this one and changing the number in the URL
If you want to see every Pokemon using the least games, then I'm pretty sure you need Emerald for Regice, Platinum for the Sinnoh starters, and either HeartGold or SoulSilver for the Kanto starters and Johto starters. I'm pretty sure that lets you see every Pokemon. Make sure you trade between Pt and HGSS (or B2 and another gen 5 game, if you have another gen 5 game) to evolve Porygon2.
I probably wasted a whole day writing this answer.
