Tierlists can be a useful guidance tool to evaluate the general strength of Heroes, but they should never be blindly followed.
Maps, synergy and counters on enemy team can influence the strength of a pick.
This tierlist is meant for players around Bronze to Gold. Pro picks don't always translate well to other levels, whereas other Heroes can really shine here.
Order is always: Support - Specialist - Warrior - Assassin.
The most powerful of each category starts on the left, then descending.
Top tier. Good first picks or first bans. You should try to aim for 1 main Tank, 1 bruiser, 1 healer, 1 specialist and 1 mage.
If you're comfortable with these, they're good early picks. Don't feel forced to take them just because they rate highly.
Comfort is more important than strength. If you're comfortable with these viable Hero picks, and they fit in your draft, they're good!
These heroes are difficult to play well, and difficult to synergise with. You're also at a higher risk of tilting your team mates in draft - even if you happen to be good at them.