Gen 1 : Blastoise, my very first pokemon, first pokemon to reach level 100 and first hm slave (Strength, Surf, Waterfall, Hydro Pump). She's from Leaf Green.
Gen 2 : with low-special-attack-hm-slave and Meganium as competitors, the title must go to the Fat Fire Rat alas. At least Cyndaquil is cute.
Gen 3: though choice but I choose Blaziken because he defeated Steven's Metagross with a single Blaze Kick in Ruby (underleveld, last pokemon for both, typical epic
anime situation). Honorable mention to Sceptile because Leaf Blade's animation was divvine in gen 3 and it was a special move. I like Swampert too.
Gen 4: unless you want your pokemon to suffer before being able to attack (boo what a cruel trainer) the only choice is Infernape. His battle against Paul in the anime is probably the best one. And he solo'd Platinum.
Gen 5: Samurott is the best of the 3, and he's just decent. If your starter had its Hidden Ability in BW/BW2 it would be Serperior.
Gen 6: Greninja, has good stats, good moves, Protean (and Battle Bond). He taught me that Sub is a great move against moles when you're fighting the worst gym leader of Reborn (yes, it's not Terra). I'd probably like Delphox too but I've never used her.
Gen 7: how can someone not like Primarina? She's adorable! And can use the Spirit Bomb! Too bad for that speed...