Looks like it's time for me to join this thread.
Regarding favorite reveal trailers, mine would probably have to go to Greninja's. I remember being a little excited when I learned this was going to have a Pokemon reveal. When the shadowed figure first showed up, I was among the many others that thought it was Mewtwo, but unlike the majority, I wasn't screaming with joy when "Mewtwo" first showed up. I just said, "Oh, Mewtwo is coming back. No surprise there since he just had a movie and two Mega Evolutions. Gee, I was really hoping for a new Pokemon to surprise us all." I wasn't all that interested in seeing Mewtwo, hence why I didn't react with excitement. I did enjoy watching the battle between Charizard and the Smash Bros all stars though, as that X/Y trainer battle remix only made things better.
When the ninja star intercepted Mario and Charizard's attacks, I was quite shocked and began to think what if the shadowed figure wasn't Mewtwo. Before long, the shadowed figure was revealed to be Greninja. I was absolutely surprised; I remember thinking to myself, "Is this a dream? Greninja, out of all the generation 6 Pokemon, was chosen to represent in Smash?" When the splash card jumped in and the gameplay footage began, my reaction became mixed with shock and happiness. Greninja was one of my top 3 favorite Pokemon at the time of the reveal (and he still remains that way to this day), and seeing him get into Smash was... amazing to see considering it was a wild card at the time regarding which generation 6 Pokemon would join the Smash roster. I loved how Greninja's moveset emphasized the ninja aspect to him, and perhaps my favorite part of the trailer was probably the moment after his splash card was revealed: I loved how he used Hydro Pump to escape the fighters as the best part of the X/Y trainer battle remix kicked in. Looking back, this reveal trailer gave me fond memories because Greninja became the very first fighter I was interested in playing for Smash Bros.
My runner-up favorite reveal trailer would probably have to be Palutena's reveal trailer. While it was obvious who the reveal was going to be right from the beginning, I absolutely loved watching that battle between Pit and Link in anime style with that epic music playing in the background (seriously, what is the name of that track, if it's from KI: Uprising?). It made me wish there was a Smash Bros anime.
Megaman's reveal trailer is a little behind Palutena's reveal trailer in terms of favorite reveal trailers. I remember wanting to see Megaman in Smash during the Brawl days, and seeing Megaman finally join the roster made me smile, not to mention the music and the action during the trailer only made things better.
Cloud is also around the top of the list since it was a reveal that no one was expecting, especially for me. When the stars first appeared, I didn't even think it was for Smash until the Smash Bros title card popped up. When they revealed the Final Fantasy title, I was thinking that perhaps this was a trailer reveal for Final Fantasy costumes (quite the odd thinking I had there). When the Cloud's model first showed up, I was thinking that I was looking at a Mii Costume... until the sword and the way the figure was holding it was shown on camera. I remember thinking, "Cloud?" My thoughts were spot on, as he was fully revealed shortly after. I was quite shocked, particularly because he didn't make any appearances on Nintendo consoles. The way he fought, though, made me interested in trying him out, especially when the Limit aspect to him was shown.
Ryu's reveal trailer would've been very high up on this list as well.... if only he wasn't leaked prior. I was very happy to see Ryu in Smash since he was a character I wanted to see in the game since my childhood days, though my hopes were shot down after Sakurai made the statement that he wouldn't include any fighting game characters. When the leak came around, I became very excited to see Ryu, and I remember trying to imagine what his reveal trailer would be like. When he was officially revealed, the reveal trailer did make me hyped to see Ryu... but I would've been much more hyped if he wasn't leaked prior, not to mention this trailer used in-game models before the gameplay footage was shown. I can understand why that was the case, but I personally wished to see Ryu's trailer using the SFIV graphics from the opening; now that would be pretty cool.