Forza Horizon 3 is their second biggest 1st party title on Xbox One... Considering it's also released on PC the Forza Horizon series has probably made more money than Halo this generation.
Halo MCC and Halo 5 made lots of money. 5 made $400m it's opening weekend and it has req packs which also made lots of money.
The $400m figure includes the price of bundled consoles, If we're ignoring the sale of hardware (halo 5 definitely pushed more systems), Forza Horizon has been a bigger game series over the last 5 years imo.
In terms of Physical XBO sales (so no 360 numbers for FH2):
Halo 5+MCC = 8.33m
Forza Horizon 2-4 = 6m
But neither Halo 5 or MCC were released on PC, while horizon 3+4 were which should easily push it at least level, and if it's not higher than Halo already, it will be as FH4 continues to sell, the above number only includes 2 weeks of FH4.
There's not really anyway to tell if we include Microtranscations and DLC, (Forza Horizon has a lot of DLC), and if we include sold hardware Halo definitley wins. But in terms of actual games sales Forza Horizon is very likely ahead, or soon will be.
LTD: PS4 - 125m, Switch - 110m, XBO - 51m
2020: PS4 - 9m, Switch - 22.5m, XBO - 2.5m, PS5 - 4.5m, XBX - 2.8m