Two Pokémon have a sentimental value to me.
The first one would be Espeon. I spent hours training my Espeon in the Silver version, and I was so happy when it reached the level 100. Moreover, as a young girl, I didn't know every Pokémon so I was agreeably suprised when me Eevee evolved into it, me who thought that you had inevitably to give it a stone to make it evolve ! So, the first time I saw it, I immediately love it.
Then, Feraligatr. Because of my silver version again, with Espeon they were my strongest Pokémon. And as a starter, he had a special place in my heart.