Many people compare Rescue Cat to Junk Synchron, Deep Sea Diva, Debris Dragon, Alien Anmonite, and Dark Salvo.
Yet, the only one that got even considered by the banlist is Rescue Cat.
Lets compare Junk Synchron and Alien Anmonite. Both are similar enough right? Summon, Revive, Synchro for level 5.
Junk synchron gives access to Junk Warrior(not always used), while Alien Anmonite gives access to Golgar(which is always used.). Each card has access to their own special synchro monster, with its own effect to be abused in one way or another.
Deep Sea Diva gives access to level 5s, but also Gishilnodon.
Debris Dragon is a whole step above, but its restricted by two clauses: It cant make a level 8 by itself, and it can only summon a dragon synchro.
Rescue cat bears the edge over these cards because of the variety of level combination that can be made with itself, and even more now with the new Fiend Diety Roar Beasts. Also, the synchro monsters summoned have no restrictions, like debris dragon.
Also, the fact that rescue cat is the only one that can be tutored as easily through summoner monk and still get its effect off really helps too.
Looking into the past of Rescue cat, it was used one time to primarity summon Level 6s, then the next format to summon level 7s. Now, its still used to summon level 9s and 5s.
Yet, all of the other one card synchros are restricted by themselves to one level range.
One of the reasons Junk and Debris works better than most of the other one card synchro combination is that they have access to a wide range of Synchro monsters, through the use of two different one card synchros.