Serperior - In my very humble opinion, this is the best designed starter of them all. A big reason for my love of Serperior is the buff it received in Drayano's hacks of B/W/B2/W2. The additional Dragon typing helped it benefit so much, and I understand GameFreak's decision not to give it Dragon typing originally as it would be overpowered, but I think it suits this Pokémon down to a tee, and it's design and it's competitive role also play a part in my love of this Pokémon.
Archeops - This is a Pokémon that has recently found a spot in my heart. It's SO poweful, competitively it takes souls as long as you have Wish support and there are no rocks on the field. It's design is really nice too and in 6th gen terms I'd love if it got a Mega Evo with only a slight boost in stats, if any, but an ability change to something like where it's attack is boosted by 1.5x once it's health is below half. I think that'd be pretty cool.
Zoroark - Zoroark is just such a cool Pokémon overall. It's design is immense, it's ability is original and cool and it's just incredibly powerful. I always get caught out while facing Zoroark as it's just so tricky making predictions for/against it being the Pokémon out against you. It's just so mischievous and fun.
Haxorus - I'm not a HUGE fan of Dragons as a whole, prior to 6th gen I thought they were overpowered. Haxorus always just seemed super cool to me, and as I kind of fell out of love with Pokémon early 5th Gen I thought it was a Pseudo-Legendary until I seen it's stats when I got more into the competitive aspect of Pokémon. It's Shiny is god damn perfect too.
Bisharp - "My bars Bisharp like a Dark and a Steel type." The design of this Pokémon is genuinely ridiculous. Again, when I first started 5th Gen I could never guess this Pokémon's type I was sure it was a Bug type lol. Such a great Pokémon overall, the power is scary as hell.