Good question.
FE11 is a much more faithful remake of FE1 than FE15 is of FE2. FE1 is super janky and the UI is hard to get accustomed to by modern standards (doesn’t do math, ranges hard to see, etc.). You can get a good sense of FE1 by playing FE11.
FE12 is a remake of FE3 Book 2. I personally am a fan of FE3, and find it to be probably the most quintessential “fire emblem” feeling game. FE12 takes some liberties and adds a lot of extra stuff, which you may or may not enjoy more depending on your preference. FE3 Book 1 is a stripped down version of FE1, which has some novelty, but is probably the worst rendition of the game.
While FE1 is prob not worth revisiting since FE11 exists, FE3 and FE12 are different enough that you can appraoch them each separately and have a different sort of experience.