I voted for the Triple Treat match, because of the 5 matches, it was by far the best IMO.
The Batista/Triple H match was boring, which was a shame because the feud and story behind it was one of the best build-ups to a match that WWE has done in this decade IMO. Batista and Triple H went on to have some solid matches afterwards, but their first match at Wrestlemania was boring and IMO one of the worst main events in Wrestlemania history.
The Cena/Triple H main event was fun to watch but not a great, classic match. The crowd was more entertaining than the wrestlers in the match. It was decent, but not amongst the best matches of either careers.
Cena/HBK was a pretty good matchup, but not the best of the five.
Edge/Undertaker was a phenomenal match, but it didn't have near the emotion that the Triple Threat matchup had. Plus Taker and Edge have went on to have better matches, such as last night's Hell in a Cell and the TLC Match from One Night Stand.
The Triple Threat matchup isn't as significant and as much of a "feel good moment" now, obviously due to what happened last year, but if you can take all of that aside and remember how it felt back in 2004 to see Benoit after 20 years of wrestling finally winning and getting a meaningful/official World Title reign, it was one of the best moments for me ever in wrestling, and I'm sure for many other wrestling fans at the time. With Eddie coming out at the end that made it all the more better. And the match itself was one of the most exciting matches I've ever seen. It was action packed throughout the majority of the match. Triple H was practically invincible at that time, and it wasn't for sure if he was really going to take the fall for Benoit or not. When he got put in the Crossface, it was such a suspenseful, exciting finish.
I would not only rank the Triple Threat matchup as the best Wrestlemania Main event of the last 5 years, I would rank it as one of the best Wrestlemania main events ever.
Of course there is the debate about whether it would've been a better match had it not been a Triple Threat match but a one-on-one matchup, but then you're dealing with "what if's." With subjects like this, it's best to deal with fact and history and what actually happened. "What Ifs" don't pertain to what actually happened and the result of what happened (aka: whether the result was great or not). And the Triple Treat matchup was one of the most exciting, action packed, and emotional wrestling matches I've seen.