I liked the 4th one and actually the 8th episode as well. Both were pretty much episodes that looked into the past and unveiled stuff form the character's histories. The finale was the expected action episode IMO. It works better if you watch the 8th and 9th together much in the same way episode 1 and 2 works better together.
Yes, episode 8 is right up there for me too and arguably the most significant given what it reveals about Wanda...as well as what it doesn't reveal about Pietro. We find out that Wanda had latent powers that the Mind Stone amplified, but it's not clear why Pietro survived the experiments and became QS. Perhaps "It Was Wanda All Along"?? - i.e., she somehow protected him during the tests and channeled the energy to provide him super speed?
Agreed on the finale - thinking of 8 as Part 1 and watching them together is the way to go.