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.
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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.
Second Yorkshire gold. They do a decaf Yorkshire tea as well. It's my favourite tea of all time, I refuse to buy another brand of black tea.
Yorkshire gold is my go-to, as a Brit. I have fancier stuff for when I'm feeling like something a little extra or weird, but for a basic cuppa tea, this is peak.
Forgot to say - I'm assuming you want black tea, but RoT and Adagio both have good oolong, greens, and whites.
I like Ahmad brand Earl Grey quite a bit
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).
I get most of my tea from a family member who sends it to me via mail. It's just regular tea in a box though.