I'm not the type to rank aspects of professional wrestling. I'm not going to waste the next 15-20 minutes of my life finding my personal order for the first 16 Wrestlemanias. However, I have no problem letting everyone know the ones I feel were the best and worst of their eras.
Worst: Wrestlemania II
Is there really any argument here? This Wrestlemania (even during its time period) was a complete abomination. It was a failed experiment, trying to utilize closed captioning in three major cities across the US to gain dollars prior to the PPV era. All we wound up getting was major delays and lots of fluff in commentating that was designed to kill time in between raising and lowering projector screens for the live audience so they could catch all three parts of the show.
Meanwhile, the card had a bunch of lackluster matches and some (dare I say) main events that had hardly any buildup unless you count the Piper/Mr. T boxing match that was still milking heat from the previous year's feud. Back then, the WWF was just learning how to make good TV by building long-term feuds with more than just their main event. As a result, the majority of the card was a real clusterfuck and was thrown together, leaning on the Hogan/Bundy cage match to be the main draw. No other matches during the card had real meaning besides the battle royal but even that match offered next to nothing to Andre the Giant.
Best: Wrestlemania III
Most people of our modern generation judge a good wrestling show by its quality of star-power, buy-rates, live attendance, match quality, overall entertainment and its main event draw. This Wrestlemania had ALL of these factors.
Match Quality - Up until HBK/Taker I, many considered Savage/Steamboat to be the single greatest Wrestlemania match of all time. This was because two midcard wrestlers wanted to show the world that the main event of a wrestling program shouldn't be the single greatest draw of the show. They knew they could put on the best "wrestling" match on the show and succeeded in doing so.
Star Power - Mary Hart, Bob Uecker, Alice Cooper, Aretha Franklin... not to mention the fact that (at the time) WWF stars like Hogan and Andre, were considered to be mainstream stars.
Buy Rates and attendance - Wrestlemania III broke every record that the WWE and Pontiac Silverdome ever held. The money made from this PPV was completely staggering, at that time.
Overall Entertainment - There were 12 total matches on this card. 9 of those matches had actual feuds with months of buildup. Haynes and Hercules had an entertaining feud over who was the true master of the full nelson... Junkyard Dog and Harley Race had a match whose loser had to bow to the other... The Dream Team won their match but also imploded as they left Brutus Beefcake in the dust... Roddy Piper wrestled in an emotional match with Adrian Adonis and was supposed to be his swan song, only to extend his career and create the birth of Brutus "The Barber" Beecake... Steamboat rose above adversity, defeated Randy Savage, marking the first time in WrestleMania history that the Intercontinental Championship changed hands, had one of the greatest Wrestlemania matches of all time, and all of this after a debilitating (kayfabe) injury... and who could forget the Hogan versus Andre feud?
If that's not entertainment, NOTHING is.
Main event draw - I don't think I even need to explain what Hogan vs. Andre did for professional wrestling as a whole, let alone that PPV.
There is no argument here. Wrestlemania III was the greatest Wrestlemania of the first 6 years and arguably the greatest of all time.