Here's my faves and least faves 3rd Gen-wise.
My favorite starter line this gen, I just had to use it when I played Ruby. Also, I love the meme it spawned. My fave Water Starter prior to 5th Gen.
Breloom is one of my fave Grass-types, and Fighting-types too. It has high Attack with really good moves and abilities (and a Poison Heal Breloom combo'd with a Toxic Orb can be hard to take down). I also love how it has a kangaroo-like body.
Electric-types tend to have awesome designs, and Manectric here is no exception. Its cry made me like it, and it's also pretty tough-looking too. Still one of my fave Electrics to this day.
My fave Psychics from this gen, I love the cuteness of the entire line but especially Gardevoir. I also love the jokes surrounding male Gardevoir, and I can also recall my first Gardevoir being male. They are still pretty good-looking Psychics I say. Ralts and Kirlia may have terrible stats but Gardevoir's awesome BST makes up for it.
There's something I love about Roselia. It's one of my fave Grass-types and also my fave Poison-type. It was also the very first Pokemon I won a Master Contest with, and is still my only one to do so.
Ah yes, the Lati's. These two are among my fave Legendaries of all time cos they have an interesting type combo and they are a good mix of cool (Latios) and cute (Latias). My fave Legendaries from 3rd Gen.
Now for the least faves:
Eek, I hate Mightyena. It has terrible stats and no effective strategy. And its design freaks me out.
Nosepass, this is one of the ugliest Rock-types ever. I hate that nose, and even worse is that it's the signature Pokemon of Hoenn's first Gym Leader Roxanne. I mean, ugh.