What would you say is Microsoft's worst game for microtransactions?
I've played Forza Horizon 4 for a while and you can buy cars in that game, but you earn enough points that they all become pretty cheap after only a few hours in the game.
The biggest issue for me was that certain in-game cars were locked behind the spin-the-wheel game, and while(thankfully) there was no way to spend real life money for more spins, it still felt absolutely shit that I could have 100+ in-game hours, hundreds of millions of in-game currency, and yet no, I can't have that Mercedes E63 AMG, because it can only be won through spins. In Horizon 3 you could just buy any car at any point, it was just a matter of getting enough currency.
Thankfully Microsoft experimented with the real money payment for sort of the "spin-the-wheel" equivalent in the mainline Forza Motorsport series (off hand I can't remember if it was 5 or 6 that was the absolute nadir), and learned enough from that failed experience (and a similar one in I think it was Gears 4 and Halo 5) that it never infected the Horizon series.
Forza Street probably. Anything with microtransactions is bound to be bad for customers but FS is probably the most criticized MS product to date (and yeah, it's a re-skinned, acquired product but I'd still count it against Microsoft since they attach themselves to it).
Halo 5, Warzone REQ packs could be a money sink for certain kinds of players. Personally, I haven’t spent a dime on the game except the initial purchase - but it really soured the community that one could spend real money on power weapons. Given that many items in REQ packs are single-use rather than permanent unlocks it can be a whale magnet as well, and I absolutely do not think game developers should be allowed to capitalize on people with addiction problems like this.
You can't buy them in-game without buying them with real money first. Once you unlock the car pack, or spend 1/3 as much to unlock that particular car, you can buy as many extras as you want with in-game money.