We can probably help you better if you specify what parameters you are looking for.
1) Do you know (or care) about the differences in play experience based on what point in the timeline you pick?
For instance, most new players will probably just jump into the 5e timeframe (and all of the 5e adventures are set during it). On the other hand, players who didn't care for the changes to the FR that happened during the 4e era are likely to want to set a game pre-4e timeline. If you want dragonborn and the 5e PHB Asmodean tieflings running around, you'll want 5e or 4e materials, since they don't exist prior to then (see the 5e Sword Coast Adventurer's Guide for rules for earlier edition tieflings, however). If you don't want them running around, you'll avoid 4e materials like the (Spell)plague.
2) Do you have a region of preference?
5e has only covered limited parts of the Realms. If you want more detail on other areas, you'll need to go to a previous edition.
3) How much detail are you looking for?
Some books have a lot of detail on certain areas, while others have a broader overview of a lot of areas. For instance, 2e had a lot of jam-packed regional sourcebooks with a ton of fluff that works just as well in 5e as it did in 2e. The 3e Forgotten Realms Campaign Setting is a good mix of both breadth of coverage and selective details.
In general, I'd say go with the 3e Forgotten Realms Campaign Setting, because it is unmatched as a one-stop setting book, and also get the 5e Sword Coast Adventurers Guide for specific time-line agnostic guidance and rules for how the PHB races and classes fit into the Realms--and the extra subclasses and subraces crunch.