Depends how well you tolerate the pacing and quality of older media.

TOS (The Original Series) is wild and fantastical in its own special way. It's not up to modern standards in many ways so I personally wouldn't recommend starting here.

TNG (The Next Generation) holds up much better. Full of space exploration and philosophical dilemmas. This is a good starting point if you want to watch by release order, but be warned that it takes a few seasons to really get good.

DS9 (Deep Space 9) picks up after the events of TNG but is mainly focused on a space station orbiting a contested planet. Less exploration, more politics and the consequences of war. Personal favorite but probably not the best starting point.

Voyager follows the crew of a ship that gets sent to the other side of the galaxy and has to find their way home. More adventurous with the writing because everything here is new, and it mostly works well. Less connected to the other series so starting here works just fine.

Enterprise is a prequel series and it's pretty divisive. Some great characters and history building, but they go hard on the "American space pioneers" theme. Just watch the intro theme. Please don't start here.

Discovery is the first series made for modern audiences and it shows. Fast paced, high action, gorgeous visuals, and woke af. It's Trek for short attention spans. It's actually really good for the first few seasons, then it goes off the rails. Start here if you want it fast and loud.

Picard is a follow up of the captian from TNG but now he's old and retired. Packed full of fan service moments. Don't watch this until you've seen TNG.

Lower Decks is currently airing. It's animated, silly, and has a lot of fun with the franchise. Great stuff here that you will appreciate more if you get the references, but if you want fun, short episodes then go for it.

Strange New Worlds is also currently airing and is quickly becoming my new favorite. It's a prequel to TOS and has found a wonderful balance between old and new Trek styles. I think this is the best place to start if you're used to modern production quality and have never seen anything else in the franchise.

There are a ton of movies you could start with as well but I'll leave it at that for now.