The best game in the Forza franchise? Well, yes. And no.
As a game, Horizon does do a lot of things right. Very right, actually. It's easy to get into, it presents itself well, it looks great, sounds great and it's fun. There really isn't much more I could ask of a game, and as such, I think it's actually better at being a game than Forza 4. That's mostly down to the presentation, I'd say. Horizon manages to be very... Well, alive. It's hard to descrive, really, but in comparison, FM4 feels somewhat stale and sterile. And that's one of the things that I always though was a big plus had over GT5, which seems even more sterile, stale and dull than FM4. The fact that the single player career doesn't turn into a huge grindfest also helps with this.
Horizon also brings some new things to the table that I, personally, think will push the Forza franchise, as a whole, forward. Most notably, of course, the night racing and offroading. Both of these have been somewhat highly aniticipated to make their debut in a Forza game - and here they are.
However, Horuzin aalso lacks things that FM4 offers - things that make FM4 great. The car list is the biggest one, in my opinion. The amount of diversity of FM4's car list are, if you're asking me, unmatched. On consoles, at least. It's undedrstandable that Horizon has to make do with less cars than FM4, but that doesn't change the fact that Horizon's car list doesn't hold a candle to FM4's. There's also the lack of tuning, which, to me, is a much more minor issue, but an issue nonetheless.
There's other stuff, oike the differences in physics, AI and such - but they're mostly minor details. I think of them as such, at least. What strikes me as pure irony, though, is that FM4 seems to be more of, how shall I put it... More of an automotive playground. Again, something that I can't really explain, but I think that FM4 allows you to do so much more, at least with the community. Granted, you can't just cruise around, but the variety of races you are able to create, at least online, if you wanted, easily makes up for that.
Overall, I'd say that both FM4 and Horizon are equally good games, but for entirely different reasons. That's ingoring the current DLC debacle, of course. Now, if Horizon 2 was to feature as many cars as Forza 5, doesn't ignore tuning and offers a race track or two somewhere in the world, along with the exact same physcs - at that point, I'd say that it's better. Until it does that sort of stuff, it's just going to complement the Motorsport franchise.
And I'd think that that's the intention of Microsoft, T10 and PG. I really can't imagine that they're out to create a game that's all around better than the Forza Motorsport games.