The Exorcist

This hugely influential, genuinely seminal horror film has quite a reputation. And it deserves it; it is indeed an excellent film.
It is not massively scary. The effects are a bit disgusting, with moments of shock, but in our modern age the tricks have been repeated and they are not particularly noticeable. More interesting is the use of Christian and sexual imagery withing the 'horror' scenes. This is still something geuinely different. It is also something that is genuinely interesting, setting the film apart from its peers.
The film doesn't operate on big tension and shocks, but on the coming together of ideas, the very substance of our beliefs. The crises of Christianity are interestingly explored, the questions, while maybe not entirely left open, are never simply reduced. The use of the young priest with this is expert. He is so much more than a cipher, he is a real person who we can explore the mysteries through. Indeed, all the characterisation and development is excellent, the acting exemplary, especially Jason Miller as Father Karras.
So this film, with its exploration through its plotting of such strands as body-horror and growing up, is a very interesting one. It is also entertaining. The plot is lively, and the action is there. One criticism is that the pacing is slightly odd; the lead up too long and the pay off too short. It sometimes feels that we head off down an alley only to turn back. But these are mild criticisms. It is a great watch.
All this intelligence and entertainment, along with a few haunting images, some interesting use of zoom-in/out, a generally pleasing aesthetic, and a few intrusions of cool music (tubular bells) make for a fine film. Rightly a classic.