Taking "sentiment" out of the equation and looking at each one as it fits into the grand scheme of things (in the wrestling business):
1. Undertaker vs HBK at WM26. While it was a memorable match and it WAS Shawn's "retirement match" it really didn't/doesn't affect anything other than tugging on the fans' heart-strings. It didn't change direction for the WWE, it didn't turn the most over wrestler in the business into a "bastard" (or affect his popularity) for "retiring" Michaels, and it didn't "impact" anybody on the WWE roster (except for clearing a main event spot for somebody to step in to). This match was pure "sentiment" as the fans were saddened to see the best worker in the business at the time call it quits. For "emotional" it gets a 10. But for "influence and importance" it gets a 2. Other than clearing the way for The Miz to become a main eventer the WWE as a business was not impacted much at all.
2. Hogan slams Andre at WM3. Hogan did the "impossible" (at least according to the WWF) and slammed an honest-to-God GIANT! That made Hulk Hogan "immortal" and an even bigger "hero" than he was before. He had slain the ultimate villain, a "fire-breathing giant", and went a long way in convincing the world's children that "saying your prayers, etc" WORKS. Increasing Hogan's status as a "hero" and "role model" helped Vince take the WWF to the whole world and if Hogan wasn't a household name by then, he sure was after slamming Andre. That slam capped off a spectacular show and took the WWF (and Hogan) to new heights and farther places. This one gets a 7.
3. Hogan joins the nWo. THE most shocking (and successful) heel-turn in wrestling history kicked off a whole new direction, and breathed new life into, a company that desperately needed it. This was the turning point for WCW, that began it's rapid ascent to the top of the wrestling business, leaving Vince and the WWF in their dust, blinking and shaking their heads. WCW got everybody's attention with this one, and practically overnight, WCW shot to the top of the TV ratings where it remained for nearly two straight years. Hogan's heel-turn stopped the WWF's dominance dead in it's tracks and put Vince's plan to eradicate all promotions on a LONG hold. Had WCW not imploded due to incompetent management and lack of supervision from the parent company, WCW might very well have put the WWF out of business. WCW SHOULD have won the Monday Night War and Hogan's heel-turn was the home run that started it all. This one gets a 10.
4. The Montreal Screwjob. Real-life invades pro wrestling. One of the biggest stars of all-time gets screwed and humiliated in public and the perpetrator uses it to create one of the greatest villains in wrestling history, keeping his company afloat and surviving until the "other" company imploded. This villain also made it possible for a new type of character to appear and prosper in the wrestling business: the "anti-hero". Mr McMahon and Stone Cold went on to have arguably the greatest feud in wrestling history, leading to the biggest boom time in the WWF/E's history. That "anti-hero" and that feud defined the "Attitude Era". This one gets a 9. It "only" helped the WWF survive the Hogan heel-turn and subsequent ass-kicking they got from WCW. It did affect the WWF as a business but not quite as much as Hogan's heel-turn affected WCW.
5. Turner buys WCW. Turner injected the funds WCW needed to compete with the WWF at their level. But WCW already had a bigger "weapon" that had foiled Vince before: Ric Flair. While Hogan was a "superhero" in the WWF, Flair was a "rock star" in WCW. Flair had the credibility Hogan lacked. Hogan was a cartoon character, Flair was a living breathing larger-than-life superstar, like a major movie star or rock star. In the south, Vince never could make any serious inroads into that market. Because of Ric Flair. The south was "Flair country" and the southern fans DID NOT WANT Vince's "cartoon show". It can be argued that Vince would have eventually won anyway, but in the south Flair was king and as long as Flair was in WCW, the WWF wasn't going to win THERE. This one gets an 8 for the money Turner put into WCW allowing them to update their production values and hire more expensive wrestlers to compete with the WWF at their level.
6. The first Rumble. Vince's "cheap shot" to try to steal some thunder from WCW. WCW had a PPV called "Bunkhouse Stampede". Vince tried to steal WCW's audience by offering the first Rumble free on television on the same night. He may have succeeded in swaying some fans but in the grand scheme of things in the wrestling business it had very little affect. I doubt fans even remember the reason for the first Rumble. The show itself wasn't anything special. The Rumble lasted about a half hour and no other titles were defended except for the WWF Women's Tag Team Titles. And Hulk Hogan and Andre The Giant weren't even on the show. This one gets a 1.