As far as Rocket League's competitive tiers go, Diamond is toward the top. It's not Champion (everyone wants to be Champion), but it takes a good bit of dedication and time to get there. According to this breakdown of the most recent competitive season, Diamond-or-better players are in the top 3.23% to 10.59% depending on which playlist they favor.
That's why some people were none too pleased with their Season 6 reward. Traditionally, rewards get more ornate the higher the rank. Season 3's four sets of wheels are a good example. However, Season 6's Diamond wheels don't stand out in the same way. Here's what they look like in-game:
via destructoid https://www.destructoid.com/rocket-league-s-diamond-players-complained-so-now-they-re-getting-shinier-wheels-488744.phtml