Good question. Last week I saw someone put spoiler tags when mentioning something that happened in the Bible, so I wasn't sure what the statute of limitations was any more.
That's a pretty decent selection. I'm not ashamed to admit to crying at the end of the Magnificent Seven when I first saw it. It is the bit with the three little boys, who earlier tell Charles Bronson that they will avenge his death and put fresh flowers on his grave, putting fresh flowers on his grave.
I have to admit that High Plains Drifter freaked me out when I saw it. However I was quite young when I did, so that was probably a very normal reaction.
I assume that you're going down the route of more traditional westerns rather than Spaghetti Westerns. I remember liking a lot of the Jimmy Stewart/Anthony Mann movies like Winchester '73, and The Searchers probably has John Wayne's best acting role (if you can look past the dodgy racist bent). Apart from The Good The Bad and Ugly, The Outlaw Josey Wales is my favourite Clint Eastwood film. It is quite a weird movie though, and I'm not sure how much it qualifies as traditional western. It is kind of revisionist.
In any case, good luck with "research". I did something similar with The Man with No Name Trilogy for a play I was planning on writing. The play never happened (or not yet at any rate), but I immensely enjoyed watching the movies.