Republic of Tea is pretty solid for people just getting into looseleaf. Also Adagio, but I prefer their non-flavored teas (the flavored ones can be kinda intense/artificial).

If you're just looking for a solid black tea and don't care too much about it being looseleaf, you could check out Yorkshire Gold. It's what I drink most days and you can get it on Amazon.

I like Ahmad brand Earl Grey quite a bit

Adagio is my main go-to, but I also go to Sloane sometimes. I also keep an eye out for small specialty tea shops recommended by friends for specific blends.

The best tea I've ever had was from https://www.teadrunk.com/, but I couldn't afford to drink it every day (might be better as a special treat, like an expensive bottle of wine).