The best episode from season one, That '70s Episode is one of the best examples of well-written Charmed, with an interesting demon storyline as well as some important family dynamics at the core.
In the episode, a demon called Nicholas tries to kill the sisters and tells them that their mother made a pact with him which allowed him to take the sisters' powers when they are given them. Whilst trying to find a vanquishing spell, the sisters are accidentally sent back in time to 1975 where they have to make sure Patty doesn't make the pact with Nicholas.
Compared with Witchstock from season six, in which the sisters go back to the 1960's, That '70s Episode is a million times better. The events which unfold in this episode actually mean something. Phoebe finally gets to meet the mother she never knew whilst Prue and Piper discover that they had powers when they were younger. Jennifer Rhodes and Finola Hughes are brilliant in this episode as Grams and Patty and the scenes between them and the sisters (in particular the diner scene) are extremely meaningful.
So many demons on the show nowadays just pop up and try to kill the sisters. Looking back on the old seasons, we saw demons that had proper motives, like Nicholas, who were determined to kill the Charmed Ones or steal their powers for actual reasons. It's sad to see how bad the show is in its current seasons compared to the brilliant writing and excellent acting in the Shannen Doherty seasons.
I absolutely love this episode and despite being one of the earliest episodes of Charmed, remains one of the most classic hours of the show.