I'm going to suggest Once Upon a Time in the West. I've watched it once, but I don't think I gave it enough of my attention and I'd like to get back to it to give it another chance. Many people consider it Leone's best film, even over The Good, The Bad, and The Ugly.
This totally gets my vote. It's been "on the list" for ages. A professor used a clip from this film (the clip available at imdb) to teach about aspect ratio and presentation. If you watch that sequence in the proper super-super wide aspect, you get it. If you watch it on pan and scan... you don't even realize there are four people in the shot... and it's even funnier because of the lines exchanged about the number of horses. Anyway - yes please for Once Upon a Time in the West. This gives me the kick in the butt I've needed for years. I've not historically been a fan of westerns. I hadn't seen many as a kid that I liked. I've seen a few of the more modern ones... and now I'm old and grumpy... and they appeal to me more.
You should join us in the viewing theater for this, if it fits your schedule! It's a good time yelling at moves with lelibertaire and whoever else is around.
I haven't seen either of those movies or really any spaghetti western. Unless Once Upon a Time in Mexico counts.
I think it basically counts as it's very much an homage to those films. I really liked The Good, The Bad, and The Ugly. It deserves its reputation though it has been a few years since I've seen it. I also enjoy A Fistful of Dollars, which is basically a retelling of Kurosawa's Yojimbo. Django Unchained actually counts as a Spaghetti Western in my eyes. Very influenced by them as well.