Do you mean in every generation?
If you look at weaknesses, Water is superior; it only has 2 weaknesses (4 resistances), and Grass can be dealt with by Ice Beam.
Fire is in not far behind; it has 3 weaknesses (5 resistances), but all 3 of its weaknesses are weak to Grass, and a lot of Fire Pokemon can learn a grass move of some description.
Grass is usually the worst; it has 5 weaknesses (4 resistances).
It's opinionated which starter is superior, and indeed what you want to use them for in battle, so I'll give my reasons.
i. Bulbasuar - normally I would put the grass type as last, but Bulbasuar has the Grass/ Posion typing, which is really advantageous. Bug, Poison and Ground are now x1, and Chlorophyll is brilliant in sun.
ii. Squirtle - rapid spinner; decent bulk.
iii. Charmander - it gains a x4 weakness as Charizard; stealth rock switch in will cripple it.
i. Totodile - Sheer Force Swords Dance. More than enough of a reason.
ii. Cyndaquil - It's my personal favourite; in-game wise, it's probably best. Decent speed, high special attack, and acceptable (barely) defences.
iii. Chikorita - good NU Pokemon, but you wouldn't use it above that tier.
i. Treecko - Unburden Acrobatics w/ Flying Gem. Even without STAB, it's still packs a punch while covering its Bug weakness. Even without that ability, Sceptile swords dance can sweep effectively
ii. Mudkip - the final Ground/ Water typing is a bit of a draw back; Grass is now x4.
iii. Torchic - Blaziken is worthless w/o Speed Boost. It's only Uber because of that ability, and it's too frail to sweep probably without it, since it doesn't have the speed Sceptile has. Blaziken isn't a bad Pokemon, but it's simply overused and overhyped.
i. Piplup - Empoleon, dual Water/ Steel type. It now has 3 weaknesses, 1 immunity and 11 resistances. High Sp. atk, access to Stealth Rock and wide enough movepool to cover its weaknesses.
ii. Chimchar - hmm another final evolution of Fire/ Fight. High speed, high attack, nice moves to choose from (U-Turn and Mach Punch are particularly useful).
iii. Turwig - in its final evolution, Ice is now x4, and most Water Pokemon carry an Ice move specifically to deal with Grass types.
i. Samurott - Swords Dance, Waterfall, Aqua Jet, Megahorn. Covers your Grass weakness, gives you a STAB priority move, what else do you want.
ii. Serperior - base 113 speed means it outspeeds most of NU (and indeed many of your in game opponents). Grass typing and movepool are a let down, but Coil, Dragon Tail, Leaf Blade, Giga Drain is a good enough set.
iii. Emboar - abysmal defences; at least Serperior has the speed to counter its threats. One STAB water move and it'll be gone.
Gen V starters are pretty much tied for me; I went with Samurott simply because of personal reasons, and Emboar isn't a bad Pokemon, but help me if Fennekin is another Fire/ Fight. I'll have to pretend that Gen VI never existed.