It depends entirely on your tolerance for and/or exposure to the different style of older TV shows. If you’re accustomed to a slightly slower pace and a markedly lower production value, I’d recommend starting with The Next Generation or Deep Space 9 and just start binging. If the kitsch of 60s TV appeals to you, The Original Series is great, just very different in style.
Once you’ve got a dozen episodes under your belt, you’ll know whether a given series is your bag or not. But you might not like TOS and adore DS9, and so on.
Be prepared for some real stinkers at the beginning, and scattered throughout but just know that’s part of having 20+ episodes per season.
If you’d rather just catch up with the most current series, I’d say start with season 1 of Strange New Worlds. You really can’t go wrong starting there.