Excluding Generation 7 Pokemon and event exclusive Pokemon
Generation I, Generation III, and Generation VI are the only generations where all non-event-exclusive Pokémon can be obtained without inter-generational communication.
In other words, all Pokemon from before Generation 7 are obtainable in their debut (pronounced DE-butt) generation and Generation 6. The only way a Pokemon can be generation-exclusive is if its debut generation is the same as Generation 6.
The list: Bunnelby, Diggersby, Skiddo, Gogoat, Espurr, Meowstic, Spritzee, Aromatisse, Swirlix, Slurpuff, Pumpkaboo, Gourgeist, Bergmite, Avalugg
Edit: Celebi and Deoxys are exclusive to generations 2 and 6, respectively. I overlooked them because they used to be event exclusive.