And I don't mean pokemon that imrpoves by new evolutions, but ones that improved by an imrpoves movepool or physical/special shift. Most pokemon did, but the most, in my opinion, would be: Chimecho-a pokemon once limitied to Psychic and Dream Eater as offense moves, now has access to Chrage Beam, Shadow Ball, Energy Ball, Grass Knot to use it's decent SpA state, a change taking it straight out of NU to the borders of BL. Gyarados-Once limited to HP Flying and Earthquake, now loses (physical) HP Flying for access to water STAB (Waterfall/Aqua Tail), Ice Fang and Stone Edge, what for the first time since makes it actually function as a water type. Espeon-Once a complete waste of a base SpA of 130, now can use Grass Knot and Shadow Ball to cover many threats. Kingler-The same story of Espeon, just physical. Arcanine-Can finally take a true exploitation of high offneses and have a true-hiting physical fire move, Flare Blizt.
There are many more, but these are just to note.
In my opinion, I would say Cyndaquil/its evos. Its level-up moveset in the G/S/C days was pretty sparse. It only learned 6 level-up attacks total, not counting Tackle and Leer, which it comes with by default. To make up for the small amount of level-up attacks it learns, the moves are spread out over a wide range of levels, which means it doesn't get to learn Flamethrower until Level 46 (which increases to Level 54 if you evolve it once, and Level 60 if you fully evolve it).
And its moveset didn't improve in the slightest in the 3rd generation. With all the new moves they introduced in that generation, they could have at least made an effort to give Cyndaquil a couple of new attacks. But no, it has the exact same moveset it had in the 2nd generation, right down to the same levels of learning these attacks (which means you'll still have to wait until Level 46/54/60 to learn Flamethrower).
The fourth generation didn't fix the fact that, stat-wise, it's still a Charmander duplicate, but thankfully its level-up moveset is a lot less sparse, and it has some new, useful moves (I particularly like the Defense Curl/Rollout combo it learns, as Rollout happens to be one of my favourite attacks).