Even though I've seen every individual episode loads of times, I've only watched the full series in order, from start to finish, twice.
Season 4 is probably my favourite for a "Go To" episode, although I like the season 5 eps too but NEVER the uber-depressing episode (you know the one I mean!!).
Season 7 is probably my least favourite for a "Go To" episode because to be honest I find it hard to tell the episodes apart. They just start bleeding into each other. Every episode starts and ends in the Summers house, with loads of new little girl characters I don't care about running around. The only standout eps for me that season are Selfless, Him, CWDP, Storyteller, and Chosen (though I limit how often I watch Chosen because every time I do, the flaws stand out more, and I don't want to ruin it).