Quick fun with numbers (not including last night's performance):
So far, the Reds have plated 427 runs. FanGraphs credits Joey Votto for 71 wRC (essentially he's produced 71 of the Reds' 427 runs), which is 17% of the Reds' offense to date. The next highest total is Brandon Philips with 59 wRC.
Any old player will score some runs for you, though. What if we were to replace him the NL median (average) 1B, who currently is Troy Glaus (48 wRC)? The Reds lose 22 runs (+-uncertainty), which would cost them about two wins.
How about compared to replacement level? The Reds currently have 23.9 WAR according to FanGraphs, both position players and pitching combined. Votto owns 3.9 of those WAR, 2nd in the NL to only David Wright. That's also 16% of the team total (and 24% of the position player total). So, if Votto was injured and the Reds instead had to get a waiver wire guy to start at 1B, they'd be ~4 games shy of where they are. Goodbye first place.
Win Probability Added? Votto has +2.9 WPA. That means he's single-handedly responsible for almost three wins above an average hitter, strictly looking at offense. There's your lead over the Cardinals.
Votto's not the only reason the Reds are winning this year. Baseball is a team game: it's hard for any one player to stake claim for any more than a handful of wins per season. But Votto's probably the difference between being in control of first place, and not. Congrats on the All-Star game, Joey.