In terms of what?
For design, probably Raticate or Tauros (although that doesn't take away from their badassery).
For competitive play I guess Farfetch'd or Seaking (especially for a fully evolved water 'mon).
Gen I happens to be my favorite gen terms of pokemon designs. While not as colorful as the pokemon from other gens, you can hardly label or stereotype all of gen I's pokemon as plain or simple. Just look at the complexity in the designs of Rhyhorn, Scyther, Gyarados, etc. I'd say the Johto starters have even simpler designs than the Kanto ones.
Now taste in designs are completely subjective anyways so arguing and trying to prove that a pokemon design is bad is just pointless...
As for originality, sure there are a few concepts which have been done before in other Japanese media like toxic sludge but the original concepts far outweigh them.
Here are some pokemon commonly labeled as "unoriginal"
Exeggcute: A group of 5 psychic seeds that look like eggs and evolve into a coconut tree. Best idea ever? Maybe to some, obviously not to others. Original? Definitely.
Voltorb/Electrode: Where has their ever been an exploding monster based off of a pokeball? I also wonder why Electrode was designed as an upside down pokeball.
Geodude: A rock with arms. Okay, but that's a rock with arms that eventually turns into a turtle.
Now complain about the designs all you want but again, it's subjective.
And it's not like other gens are free from unoriginal concepts like garbage which is often considered "original."
Not really.