I can watch em all, any time, except STD and Star Trek stubborn regrets, aka Picard.
TNG has the best episodes but also has a lot of dull stuff, especially the first two seasons. And there is Wesley :/
TOS second, plenty great episodes, yet mostly in 2nd because of nostalgia. I grew up watching TOS.
Voyager comes next, exploration is what it's all about. No boring diplomatic missions or Troi's mother courting Worf etc.
Enterprise, more exploration. I'm not that big of a fan of long continuing story lines but it was done well in Enterprise, much better than the Dominion nonsense.
Deep space nine, some good episodes but feels more like a soap opera than star trek at times. Plus Cisko the prophet whatever, yuck. Without Garet and Julian I wouldn't have bothered to finish it.
Star Trek Discovery just makes me angry while watching it and Picard has me banging my head against the wall. Why.
Star Trek needs to go back to exploration.