3. Piplup -> Prinplup -> Empoleon
I’ll be honest: the only real thing I like about this evolutionary line is Empoleon’s one-of-a-kind Water/Steel typing. It is one of the few Steel-types resistant to Water, the others being Ferrothorn and Dialga, and the only one simultaneously resistant (4x even) to Ice. Although its typing comes with letdowns (namely being weak to Fighting and Ground, both of which are used often by physical attackers, which Empoleon does not favor), it has some interesting utility options in Scald, Stealth Rock, Defog, Knock Off, and…um…Yawn? It also has decent bulk and a solid Special Attack stat, unfortunately let down by its poor Speed. At least it’s no slower than Torterra, I guess…?
2. Chimchar -> Monferno -> Infernape
Don’t worry; I’m not going to name off all the 4th gen starters. I’ve said it once and I’ll say it again: I don’t discriminate Pokémon by their generation of origin. That being settled, this Pokémon was my first starter choice in 4th gen for two reasons: 1) it’s a monkey, which appealed more to me at the time than a penguin or a turtle, and 2) its fully evolved form has super-effective STAB on both other fully evolved starters (although the same can be said about Torterra and its Ground STAB). Back in my first playthrough of Pokémon Pearl, I slowly but surely leaned towards giving it a purely physical movepool due to its pugilistic nature (although I made Grass Knot an exception because of its ability to hit Water-types), but every part of the evolutionary line has the same Attack as Special Attack. It has some interesting offensive options, namely powerful STAB for both sides of the spectrum, U-turn to damage and switch in case of a dire situation, and Grass Knot, Thunder Punch, and Hidden Power (Ice) for coverage. It also gets boosting moves for both its attacking stats (Swords Dance and Nasty Plot, although it is too frail to use them) and utility options in Stealth Rock and Taunt. It may not be the original Fire/Fighting type (that would be Blaziken), but it is, in my opinion, the best (although not tier-wise, of course).
1. Froakie -> Frogadier -> Greninja
Like pretty much all of my favorite things, this was not a love-at-the-first-sight decision. In fact, when the starters were first revealed, I was leaning more towards Chespin than Froakie, but when I got Pokémon Y, something in my mind told me to take the frog. I don’t know what it was, but I sure don’t hold that against it. Even though its base form looks meh, as it evolved, I felt a sense of, “Hey, this guy’s really a ninja…” It’s not that I love its appearance to death, but hey, that fox wizard and um…squirrel? warrior aren’t much better off in appearance if you ask me. Another thing: realizing that the three starters formed a trio of wizard, warrior, and ninja, I thought back to the characters of Log Horizon: Shiroe, Naotsugu, and Akatsuki. The fact that Akatsuki is my favorite Log Horizon character is further manifestation of why I prefer Greninja over the other two starters.
Competitively speaking, Greninja is one of two Pokémon with access to Protean (the other being Kecleon) and undoubtedly the best. It is the fastest of the starters with base 122 Speed (which is incidentally faster than most of the OU tier), it has a decent Special Attack stat of 103 and an average Attack stat of 95 (which are more effective than they look, factoring in that it gets STAB on everything), and its offensive options are far from limited. Hydro Pump and Ice Beam are on every set, while U-turn, Dark Pulse, Extrasensory, and Hidden Power Fire were options for preliminary X&Y sets. However, the release of ORAS and its accompanying tutors gave it a nice new tool in Gunk Shot, which allowed it to deal more damage to Azumarill and Clefable than any other move while simultaneously providing it resistance to Fairy. Interestingly, it also gets Spikes and Shadow Sneak to take advantage of the weaknesses of its original typing (Electric and Fighting, respectively). There are also gimmicky physical sets, but those just do not work as well as their special/mixed counterparts. All in all, it’s one of the best starters competitively.