I go for manhattan project. I'm doing so in my current game, built manhattan project in 1902.
Little bit of back story. In the early AD's, Russia and I (celts) formed an alliance that has lasted uninterrupted all game. Together, we've partitioned Siam's homeland (Siam is still at large as a rump state,) knocked polynesia down a few notches, plundered rome (another entrant to the rump state club. Egypt and I split it up,) and just generally acting as one diplomatic entity. We're numbers 1 and 2 on the scoreboard now, and russia is slightly ahead of me in tech.
It's 1906. Russia has finished the Apollo Program, with me having finished the Manhattan Project four years earlier.
The only way I am going to win this game is by nuclear-backstabbing my longtime ally. I've got quite a few battleships on the ready, most of them having both range AND logistics (water map.) I'm working on a carrier, and I've talked Catherine out of her Uranium. This gives me 8 atomic bombs to work with. I know I'll have strategic resource penalty, but I'm mostly looking for the widespread destruction as opposed to damage done by the nukes. If I can't conquer her lands, I at least want to cripple her. After this, I'll send in a naval strike team at Moscow, and launch a small land invasion.
I'm mostly banking on the widespread destruction caused by the atomic bombs to enable me to invade and more easily conquer her technologically advanced nation. She's probably running on around 60's tech, and I'm stuck on 40's tech.
But yeah. Typically in cases where you and the big dog are good friends, manhattan project will save your butt. They go apollo program, you beeline manhattan project and blow their asses back to the classical age.
But yeah, the entire game now hinges on me being able to severely cripple russia in the span of about 30 years. I have a feeling I'm about to get into a long, bloody, multi-party war. With Russia out of the way, no one will be able to challenge me in victory, apart from Polynesia who I've had no problem subjugating in the three wars we've fought.