Can you think of any other game on PC that has knocked these two off the top in the last, well, whenever? I bloody well can't.
It undoubtedly won't last forever, but right now the most popular game on Steam by some considerable margin is Fallout 4. Typically that's the domain of Counter-Strike: Global Offensive and Dota 2 -- the two games have been making up more than 40% of what's being played on Steam at any one time for quite a while.
But as of the last day or so, Fallout 4 has knocked both of those games off the top spot. It's worth noting that just about everyone who owns Fallout 4 on PC appears to be playing right now, whereas there are a greater portion of people who play CS:GO and Dota 2 overall.
Steam Charts says just over 420,000 people were enjoying Bethesda's new gargantuan RPG just after the turn of the hour, with the game's all-time peak currently sitting at 444,825. That's nowhere near Dota 2's peak of 1,262,612 players, but then there are 100,000+ more people playing Fallout 4 on PC right now than there are Dota 2.
That peak makes Fallout 4 the third largest game of all-time on Steam, overtaking GTA 5, Left 4 Dead 2, Terraria, Borderlands 2, Total War: Rome 2 and Team Fortress 2.
In comparison, the other major franchise to launch in the last few days is struggling badly on PC. There's only 36,533 people playing Call of Duty: Black Ops 3 on Steam right now, with the game's peak at a miserly 63,681 (still in the top 10 list for Steam players, but well behind CS:GO, Dota 2 and, now, Fallout 4).