Ertl always was just an entry level brand, nice looking but most of their cars are still stuck in 1990, model-tech wise.

Their Authentics are awesome, and great value for money.
They can fork out really impressive stuff.

Since the early 2000s US car collectors usually went with Lane, GMP, Highway61, and most of these molds are labeled ACME since around 2010.

Greenlight forks out some nice simple models, too.

For non-US stuff (=the biggest part of the diecast market), high-end sedans, supercars and such, and for state of the art check out models by MotorHelix, LCD, Almost Real, brands like these.