is champion.gg a good website to use

It lacks a lot of data, so no I personally don't like it. I prefer Lolking.

It's decent to see common builds and runes/masteries

unfortunately it aggregates data from plat+ games, would be better if it had an option to show only diamond+

So what would you guys recommend for sites?

It's a good enough basic resouce until you're comfortable on x champion to make your own decisions. Combine multiple sources for the best results imo