Well, it
was, in my opinion (whatever that's worth), going to be
the Road to Doomsday, but that was cancelled.
Far too ambitious, I knew in the back of my mind it was destined to fail, but I was behind it 100%. Was going to be from the late 1700's (yeah, I know...) until modern times. Would have been "the ultimate" mod that, in all honesty, will never exist (it's a Catch 22, the Fountain of Youth).
Currently, the big ones are
New World Order (a Cold War mod),
the Total Realism Project (an improvement over the A.I.),
Fallout Doomsday (a Fallout scenario, which sounds weird, and I was skeptical, but it is amazing),
Mod 1914 (A misleading title, starts in the late 19th century and leads up to WWI. No longer active, unfortunately, but used to be my favorite),
The Bonaparte Legacy (alt-history mod where France takes the dominant position prior to WWII over Germany),
Arms, Armistice, and Revolutions (A mod I feel should have been incorporated when the game came out, as it was alluded to going to be there. The goal is to seamlessly link the WWI scenario to the main scenario), and lastly
Kaiserreich (another alt-history mod, and probably the most popular one here, where Germany wins WWI and the implications of this are expanded upon).
Now, as for what's best? Well, what suits your taste? There's a lot out there, and I really don't think there is a "best" one. Some definitely put more work into it their mod than others, but I really don't think any are purely superior over all others. That's what I like the most about the work here. Heck, New World Order is a one-man mod, and Bizon (the creator) is closer to completion (or, has the most updates) than almost everybody else.