There is no "worst" so to speak. Although there are a few bad ones. Masamune, Twinstars, Anubis, and Kubiluga. They all have their uses. Just because it's replaceable doesn't mean it's useless. Unless we're talking about Eggy cat or Killer.
Oh, that's a hard decision. I think It would be my first uber, hayabusa or maybe even King Gamereon who carried me through Sunfish Jones. I really like octopus and can can though. It's really hard for me to narrow down one. So here's a few, C.psycho cat, Paris cat, ramen cat, necro-dancer, hurricat, dread pirate catley(first rare), all the li'ls, bahamut, megidora, super galaxy cosmo(I wish I had him), C. fish(first crazed), and my newly recieved apple. All the cats are so different and unique. It's hard to choose. Also, they're not only favorable for functionality, I also like aesthetics of the cat as well.
We can be friends, and don't worry, I was just where you were a while back. I remember seeing wuffa's "beached mammals" guide when I first started out. I said to myself "I'll probably never get that far" and just look at where I am now. You'll be able to make tons of progress. Battle Cats is about waiting. It'll take time to get far in the game, even I ask people for advice. Just ask me and I'll attempt to answer it the best I can.