Well...
I love Slash, my current PMD2 partner. Since he's a Totodile, the fact that he's so whiny in-game is hilarious. C'mon. Imagine a whiny Totodile. Does it not amuse you?
Plus he may die pretty fast, but he's a faster leveler than Flea the Munchlax (me) and often saves my Gummi-hogging rotund self with his vicious physical attacks. For such a whiner he's got awful sharp teeth... And Ice Fang puts those chompers to good use.
My first PMD2 partner, Aurelius the Treecko, seemed less whiny and more.. well, depressed. Like, all the time. I felt like Tiberius could never say anything depressing ever or Aurelius would go be emo on a cliff all day. I know they use the same lines, but it's different to see a Treecko say it than a Totodile. Though I do remember him being pretty useful compared to my Chimchar avatar, Tiberius, who was dying all the time.
In PMD1 my partner Chu-Ru the Charmander was my bestest friend forever, no questions asked.