I guess if I had to pick, I'd say Ivysaur, but I could always just play Brawl if I really want to play as anyone from Brawl.
That's the nice thing about cuts - even if they don't make it in the following installments; it gives you reasons to revisit older ones.
Most Missed: Snake
Most of my Sm4sh speculation consisted of me trying to find reasons Snake could return. His play style was super unique and awesome - exemplified his sneakiness. Metal Gear is one of my favorite gaming series and Snake being in one of my other favorite gaming series is an instant win for me.
Think that if mas DLC is announced and released around the same time as MGS5's release, we have a HUGE chance to get to see Snake/Big Boss. They promoted MGS4 in Brawl, makes sense they would do it again (as they don't care - it's cross promotion so both win).
Plus Keifer said he was doing a Mortal Kombat character voice....yet we don't have a character know to us that uses his voice and MKX is pretty close to release. Possible he just knew it was a fighting game and thought it was Mortal Kombat, when it could be, in fact, Sm4sh (Solid Snake Alt Skin ala Alph)
Most Shocking: Ice Climbers
I was the most shocked by the IceClimbers being cut. At the time I really thought they would be shoe ins because of their unique playstyle and they returned from Melee to Brawl (without getting cut). I just felt like after those two games they have become a staple characters of the series - would be like having Falcon cut in the next game.
Maybe we will see them back - if they worked on the WiiU version and they have the chant in the 3DS - Maybe the new 3DS will have the strength to have them function normally.