So, here's the thing. I voted for Forgotten Realms as "vanilla" (in the sense of plain), but I don't think that's quite right.
FR is distinctive in the sense that it is the "base" setting in 5e. It also has the most thoroughly mapped out lore and world, along with numerous sub-settings that have been subsumed into the FR whole (Al Qadim, Kara Tur, Underdark, Maztica etc.).
In other words, FR provides everything you could ever want in a setting. Tons of lore, tons of crunch, a whole world, and multiple different distinctive areas. Of course, this is the same problem that many people have with it- many players are familiar with the lore (through novels etc.), there is the feeling that the players don't matter given the exploits of so many known and powerful NPCs, and the world is largely known.
Greyhawk, for multiple reasons, is different. There is not the same level of lore or crunch, and the "setting" is only one part of one continent, with the rest of the entire world being unexplored rumor (and not canon). The familiarity most people have with it are from names of things from the past that aren't present, and the general vibe is one of "uncaring," in the sense that the powerful NPCs are pursuing their own agendas and don't care much about the players or, for that matter, the world.
Dragonlance is completely different, again. Traditionally, it has been presented in the context of the heroic railroad - the War of the Lance. The setting certainly exists independently of this (and is interesting), but there has always been such a strong tie-in with specific stories it is often difficult to disentangle them.
All of this is, of course, IMO. None of this is something that I would necessarily call vanilla; someone who loves the lore of FR wouldn't think it's vanilla, just as someone who loves the distinctiveness of GH wouldn't say that either. I certainly don't think DL is vanilla.
For that matter, good vanilla is amazing. Probably the best flavor. You can take you chocolate and shove it down your own pie hole. Ima enjoy some high class vanilla, thankuverymuch!