This question really depends on what you're looking for as an investor.
If you're looking for exposure to gold with the lowest risk (out of the ones you listed) Franco Nevada would be, in our opinion, the best option. Primarily because of the fact it's a streamer. This is exactly why it is a Foundational Stock and not a producer. If you're looking for what exactly a streamer is, just head to our Foundational Stocks and have a read of Franco Nevada.
In terms of producers, Kirkland Lake is a gold pureplay. In fact, Kirkland Lake and Kinross Gold are the largest gold pure-play companies in the country. Whereas a company like Wheaton saw just over 60% of its revenue come from gold production in 2020 thus far. The remaining revenue coming from silver and palladium.
The bulk of Barrick's revenue does come from gold, as its copper production is relatively low, accounting for only around 7.5% of production.
Each one of these plays has unique characteristics.
For someone looking for an undervalued gold production pureplay, in our opinion that's Kirkland.
For someone looking for lower risk, albeit lower reward exposure to gold, Franco Nevada.
For someone looking for a mixed production company that has exposure to a variety of metals, Wheaton.
And, for someone looking for a blue-chip option that can be seen as more reliable, Barrick Gold.