Are you planning on proposing in
Disneyland or DisneySea?
I'd consider proposing in DisneySea for a number of reasons, including the abundance of restaurants and bars serving alcohol (which may help with working up the courage and hopefully celebrating a positive response), the fact that it has the best theming, the fact that it has quite a number of quiet areas and the fact that it appears to attract a slightly more mature crowd who would be more respectful if they happened to intrude upon your magic moment.
If you really want to propose at Disneyland, then I think somewhere around Critter Country, Westernland or Adventureland is going to be your best bet in locating a quiet / secluded spot which is also scenic. But, with the new Junior Woodchucks area opening soon, that may attract more people to the general vicinity.
Somewhere on the paddleboat / riverboat (if it's quiet) or on Tom Sawyer's island is probably going to be your best option, particularly somewhere on the island. If you find a good spot, you may even have scenic views across to the castle in the background (although I can't quite recall whether there is anywhere on the island high enough for such a view).
If you were happy to propose at DisneySea, then you have a LOT of options. There is the Venetian area or the Venetian bridge across to Mysterious Island. You could even try to request a "private" gondola and propose while on the gondola (although I'm not sure whether the cast members would permit this or whether they would necessarily understand your request). There is the Fortress Island, with plenty of battlements. There is Magellan's restaurant, which is a beautiful restaurant.
My best recommendation would, weather permitting, be on the upper, forward deck of the SS Columbia. It offers a great outlook across the park, including the volcano, and it never seems to be particularly busy. There is quite a big area, so even if there are other people milling about, you should be able to find an area to propose in relative privacy. And you can work up some Dutch courage and / or celebrate the engagement in The Teddy Roosevelt Lounge.
Good luck!