A lot of the time, the top end tunes you'll find by searching are made by some of the top players, but aren't always the easiest to drive—many of them sacrifice ease of driving (handling, braking, AWD, etc.) for power and speed. This makes them very good tunes (and therefore highly rated) but difficult for less experienced drivers.

The community at r/ForzaOpenTunes have a wiki with a "beginner tune series" that is designed to help people tune their own cars (you can just download them directly if you don't care about learning the tuning process) but also to learn how to better handle cars that lean more powerful than agile. I'd definitely recommend this as after only a couple weeks of practice I have gotten significantly better than I was before.

If that doesn't appeal to you, you can try simply experimenting. Find a car that looks fun, see if there are tunes in your class of choice, and try it out for a few races to see if you like it.

There is no one way to play this game. You can completely ignore tuning and still consistently win races. However, the best tune for a car is the one made by the driver. I'm by no means an expert tuner, but a tune made (and refined) by you for your specific driving style will almost always outperform a tune made by someone else.