Mine would be a tie between Butterfree, Venusaur and Raichu.
Butterfree: The nostalgia of Butterfree from the original anime's first season, Plus, Butterfree's move pool is immense. Poision powder, Sleep Powder and Stun Spore all at level 12, amazing Pokemon and easy to train and make powerfull, plus later Psychic and Confusion.
Venusaur: Since I first started playing the original Red and Blue, I picked Bulbasaur. My freind's made fun of me saying it was the type advantage over a the first two gym's. They all changed their tune once I got a connector chord and was shutting down all of their Pokemon with venusaur's Sleep Poweder and Solar Beam. I love his Move Pool as well as Butterfree's.
Raichu: The first Pokemon I loved, Lmao. When I first saw Pikachu, Thought he was an Amazong Pokemon, The one morning before school seen the new Pokemon episode 14 Vermillion Gym Ash Vs Lt Surge, Changed My life, Raichu has a desighn that look's both sleek and strong. Plus with moves like Body slam, Thunder and Shock Wave where can you go wrong, Raichu can all around throw down if trained, Pkus amazing Stats.