I guess I will play Devil's Advocate a bit in this thread. The Undertaker's Streak was impressive by the numbers alone. It was one of the biggest angles the WWE ever had built up. Do I think Lesnar should have ended it? No, after the crap fest he and Goldberg had ten years earlier. I mean, I had no problem with him leaving, but to intentionally have a crappy match on the grandest stage was inexcusable. However, The Undertaker has a ton of respect for both Brock Lesnar and the Sport of MMA. In his mind, what better way to end his streak than a former champion in both WWE and UFC? Also, keep in mind that the Undertaker has never made Lesnar submit or pin him in a Singles match. Yes, I know he pinned Lesnar in a thriple threat match but never in a one on one match, so that is another thing people should keep in mind.
Remember when Lesnar lost the UFC title and he and The Undertaker had a brief encounter? The Undertaker asked Lesnar, "You wanna do it?" There were already plans for Lesnar to come in and face The Undertaker at Wrestlemania (although it was a few years ago).
I also disagree with Cena or an up and coming superstar ending the streak theory. If Cena ended the streak he would be one of the most hated people on the face of the wrestling planet, and that would cost the WWE money in T-Shirts, Toys, etc sales. Also, why would a younger wrestler (who isn't a former World champion) defeat The Undertaker at Wrestlemania? That should have been (and was reserved) for an established badass who was believeable. Lets say, Reigns ended the streak and then a year later he decides he doesn't want to wreslte any longer, doesn't get over like planned, or gets injured out of wrestling? Then the WWE wasted the streak for nothing. This was the best route to go (although I still don't agree with it) if the streak were to have ended.
Now to answer your other questions. My least favorite streak matches were against Bundy, Bossman, and Henry. These matches were just slow and thrown together to give The Undertaker something to do for Wrestlemania. In my opinion, the streak was just something that happened by chance because there were a number of times Vince was going to have The Undertaker lose at Mania, including Wrestlemania 8 (before the face turn). There were even talks of Bundy going over Taker at Mania as well. What a huge mistake that would have been if that had happened! Not just from the streak but from The Undertaker basically losing to a glorified attraction jobber who was past his prime.
My favorite streak moment was the match with Ric Flair. Oh man what a match that was! Especially with the AA Spinebuster out of no where!
My overall thoughts on the streak is that it happened by chance and over time grew into a main attraction. In all honesty, it just gave The Undertaker a match at the end of the streak like it did at the beginning. Remember most of his matches occurred because he wasn't involved in a fued and was given an opponent, and a few of those times he wasn't going to go over in the match (Sid (8), Bundy, Nash).
And no, I wouldn't have ended the streak, but at least now we can probably get a one on one match between two other guys at Mania, rather than, who is it going to be taking the tombstone this year.