I think the Bulbapedia timeline is pretty accurate.
FRLG and RSE have to occur at the same time because you can freely trade between them, also Blue mentions that you can't catch all Pokémon in Kanto + Sevii Islands, which means Hoenn already exists and the adventure in Hoenn is already taking place.
DPP has to be after 1st and 3rd gen because of all the new evolutions that were discovered. It's also very close in timeline to HGSS, because of the features in HGSS where you can catch Sinnoh Pokémon with the radio or in the Safari Zone.
What happened in Hoenn between these three years is up to speculation, though. Steven calls himself "the Champion of the Hoenn region", but we know that Brendan/May beat him in RS to become Champion themselves. It's possible something happened that they went wandering around like Hilbert/Hilda and left the Champion position to Steven. And the fact all of the Weather Trio is catchable in HGSS makes me think Emerald is the true canon for 3rd gen, because at the end of the story Brendan/May still didn't catch all the legendaries (you can only catch them post-E4), whereas in RS it's presumed you caught Groudon/Kyogre.
DPP and HGSS take place at roughly the same time, despite Jasmine's appearance in both games. The key is the red Gyarados. In DPP, the red Gyarados already exists, which means Team Rocket's plan of forcing evolution is already underway, and such a thing obviously wouldn't go unnoticed.
The Unova games take place many years after 4th gen.