Man, people here have a very different sense of "A long time" on YouTube. I was hesitating to include TimeGhost because I've only watched them for about eight years.

For stuff I've been watching for less long, the creators that come to mind are:

Timeghost - they do history documentaries, focusing on chronological "real time" content, such as their current series, the Korean War Week-by-Week

LEMMiNO - does deep-dives into supposed mysteries. His older stuff isn't great, but shifted towards higher-quality long-form content about 8 years ago, and he does a good job on them.

AHOY - does documentaries on the history of games, and adjacent tech and culture. Not usually super deep, but well written, produced and animated.

Desync - makes stupid gaming videos, usually abusing bugs or just being silly. Doesn't post often, but on the rare occasion he does, it'll be well produced and funny.

For stuff more at the age I was thinking of originally, 10+ years:

John and Hank Green - cover a whole range of topics, although nowadays, it tends to be a mix of advocacy, science, philosophy, and just sillyness.

The Yogscast - Just a plain old gaming channel, albeit with unusually frequent and higher production quality videos. Nothing extraordinary, but consistently good and fun, despite the fact that they've been making videos for nearly 20 years.

Tom Scott - short video essays, explainers and documentaries on a variety of topics. Primarily, videos on different interesting places.

TomSka - The ASDF Movie guy. That said, just as good or better are his other skits, and now even his long-form content and videos essays.