Insidious 1 and 2 are my favorite horror movies hands down. I feel like James Wan creates an incredible atmosphere and builds tension throughout the entire movie. Another thing to note are the jump-scares. Yes, like other films the audio will sometimes drain out and blast you at something spooky or unexpected. But unlike other films in the genre, I felt the visuals actually warranted the loud music sting. The plot was fun and original, with both films having setups and payoffs for each other. Overall, I think they are a great watch, and I wish I could go back and experience them for the first time again.
I wouldn't really recommend the other two. There's a bit of "lore" you can get if you care a lot about a specific character. But without James Wan directing, they feel very average and don't hold up to the quality I see in the first two films.