Here's an extremely brief break-down of each starter. If you want more information on each placement, feel free to ask in the comments. This is considering starter performances within their respective regions where they actually play the role as your starter, so this won't consider encounters like XY Charizard or USUM Empoleon.
S/V Tier: S/V give the player so many free resources and powerful items that you could make an argument that over half the dex belongs in the S-Tier since the boss fights are incredibly easy for the most part.
Venusaur: Busted in the original kanto games and borderline S-Tier in FRLG. More consistent placements than the other starters at the top.
Infernape: Very good in the Gen 4 Sinnoh games, but does drop off a bit in BDSP due to the elite four difficulty spike. Still great in both games though.
Feraligatr: Without Substitute, it doesn't reliably solo HGSS, but it's still very strong in those games regardless. However, it is a bit worse in the originals due to a lack of set-up outside of Rage.
Serperior: BW2 Serperior is debatably a top 5 starter in the series, but it's BW performance does drag it out of the S-Tier overall.
Swampert: It's great in RS, but every game after that sees it fall off in the late-game and get outclassed by a plethora of better options the game gives you. Still a very consistent starter overall and is far more beginner-friendly than the ones placed above it.
Blastoise: Swampert if Ruby and Sapphire didn't exist. Great encounter, gets a lot done, but still outclassed by better water types. However, that doesn't mean it isn't good in it's own right.
Empoleon: A great Swords Dancer in the gen 4 sinnoh games that unfortunately sees a slight decline in the remakes. Still solid in every game it's in, and vastly outshines Infernape in the late-game of Diamond and Pearl.
Primarina: Great in SM, but gets a bit worse in USUM due to changes such as Hala being removed from the league and Totem Lurantis always calling an ally after the first turn.
Cinderace: Gets a lot of good attacking options early and has a solid spread of matchups, but doesn't have an X-Factor that pushes it over the edge.
Blaziken: A great E4 option in the gen 3 Hoenn games and a Mega Evolution merchant in the remakes. Its gym leader matchups hold it back.
Meganium: Swords Dance carries it in HGSS, but it's a lot worse off without it in the originals.
Torterra: Solid performance in the Gen 4 games, but it really drops the ball in the remakes where it doesn't evolve before Maylene and becomes a lot worse into the E4.
Samurott: Great in BW2, but mediocre in BW.
Emboar: Great in BW, but mediocre in BW2.
Typhlosion: Never a bad option, but doesn't have any standout matchups or performances that elevate it out of B-Tier in either generation. Rollout in Gen 2 and Choice Specs + Blaze in Gen 4 sound great on paper, but the former is inconsistent while the latter fails to reach several important damage thresholds.
Rillaboom: Same as Typhlosion. No standout performance and its movepool in the context of a nuzlocke is extremely bland by SWSH standards.
Delphox: Has the displeasure of learning no good moves until the mid-late game. As a result, it struggles to do much of anything before fully evolving, and faces competition from stronger fire and psychic types.
Inteleon: Bad early-game matchups and a limited movepool hold it back quite a bit in comparison to other starters. There's also a plethora of strong water types available that it has to compete with, which doesn't help.
Incineroar: Just doesn't do a lot throughout its games outside of beating Totem Lurantis. It has good tools like Will-O-Wisp and U-Turn, but it plays more like a stat-stick than anything when it fully evolves. A decent league performance keeps it out of the C-Tier.
Chesnaught: The Kalos starters really get shafted by bad level-up learnsets and TM distribution. Chesnaught matches up quite poorly into most gym leaders and doesn't do a whole lot in the E4 to make up for it either. It's a decent option for stalling though and it does shut down Lyrandre's Mega Gyarados, so it's a high C-Tier placement.
Sceptile: A victim of 70BP Leaf Blade and the fact that it was designed to be a special attacker despite Swords Dance being its best boosting option. Underwhelming in the Gen 3 Hoenn games, but admittedly a bit better in the remakes.
Greninja: In the same boat as Delphox where it gets no good moves until the late-game. Unfortunately, it has a worse matchup spread due to its typing and faces more competition due to being a water type.
Charizard: See PChal's Tier List video regarding why it's terrible in FRLG. It's not as bad in the original Kanto games, but it's still far from being good due to its matchup spread and lower special stat.
Decidueye: Not exactly horrible in SM, but far from being great. In USUM, however, picking this thing is an act of self-sabotage. Not only are you giving the champion their strongest team, but you're also picking a starter that has literally ZERO winning matchups against relevant bosses due to its terrible typing, stat distribution, and movepool. Decidueye does not solo any boss fight and is only able to contribute in the E4 as a Feather Dance spammer. It also doesn't help that Trevenant is a nigh-guaranteed encounter that overshadows it.