I am going to exclude any shows for which I had previously read a manga or novel version of the story. There are a couple shows I can think of where I knew after episode 1 that I was in for a good time:

• Kino’s Journey (2017) - The first episode is excellent and really sets the tone for the kind of show you are in for. It’s really, really good.
• Heavenly Delusion - I went in completely blind to this show and was blown away. I was instantly hooked and after the show ended I binged the entire manga, keeping up with it to this day. The OP is excellent as well. Unfortunately no season 2 announcement though.
• Little Witch Academia - I love LWA and I fell in love with the world and characters right from the beginning.
• Konosuba - As somebody that has read/watched way too many isekai over the years, this show hit the humor sweet spot for me basically as soon as Aqua entered the picture.