I'd say that it's the latter two that are really important. For all its crazy imagery and plot elements, Evangelion is really a story about its characters. They're all complex people who are ultimately just trying to be happy but don't really know how to go about it. It deals with very real, human pain and difficulty and it's genuinely relatable and touching. But as depressing as it is, it still ultimately ends on a positive message and that's easy to appreciate.
On top of all that it's just really cool. The designs for both the Evas and the Angels are excellent and the fight scenes in particular are amazing. They're varied, memorable, and disturbing. They work really well at supporting the overall tone of the series and individual scenes. The religious symbolism and naming are included for no reason besides that they're cool, but the fact remains that they ARE cool.
What I'm getting at is that even if you don't care about the deeper themes or other more difficult aspects of the show Evangelion is still really good. That's another thing that makes it so impressive to me.
It's not perfect, it's a bit of a mess and some of the plot elements don't get explained. At all. But it's such a well-directed, artistic work that any flaws seem irrelevant, vastly overshadowed by how much it does right. It's very human, exciting, and occasionally profound. If you have any interest in anime at all, you really should give it a shot.
