Lemme see...
1. Anything not named "Chansey" that got an evolution in GSC. There's a very good reason why the likes of Crobat, Steelix, and Kingdra were created, and all you have to do is use Golbat, Onix, and Seadra in RBY to find out why.
2. Beedrill. Don't get me wrong, Butterfree sucks pretty bad, too, but it's at least quite useful in the early game with Confusion/Sleep Powder. Beedrill, on the other hand, doesn't even have that much going for it. Awful stats and no useful moves, whatsoever.
3. Pidgeot. Sure, it looks kind of cool, but its stats and movepool are anything but. Spearow's line is not only stronger, but also grows faster (you get Fearow at L20 and Pidgeot at L36... 'nuff said), and by the time Fearow starts to fall behind, Doduo is usually right around the corner. There's no need to use Pidgey's line, ever.
4. Anything that can be caught with the Good Rod. In a way, there's actually even less of a reason to obtain the Good Rod than the Old Rod. The Old Rod catches nothing but Magikarp, yes, but Magikarp evolves into an excellent Pokémon. The Good Rod, on the other hand, catches nothing but... Goldeen and Poliwag. You know, a couple of generic, horribly outclassed Water types?
5. Pikachu in Yellow. No Light Ball. No evolution. First gym uses Rock/Ground types. Just. Shoot. Me. Yeah, it's cute, and it follows you around, but that's about it. Have fun later in the game barely making a scratch with Thunderbolt while dying to Tackle.
6. Any novelty/gimmick Pokémon. Aka, Ditto, Farfetch'd, Lickitung, etc. Obviously, these Pokémon all suck and aren't worth the time. (Although, Farfetch'd actually makes a decent HM Slave.)