I don't really think that sweeping generalizations can really be made to overall effectiveness just due to the fact that certain builds peak sooner than others, due to either powerful armor effects or weapons way above the power curve that last for an unnaturally long amount of time. By the end of the game though, I'd say everyone is a solid "A" on this sliding scale, but instead of nebulous grade ranking, I'll just give writeups.
These writeups are my own opinions and should not be taken as law or whatever, and I'm mostly going to be basing these more on vanilla mid-lategame performance; everyone is broken with Moon Lord gear/armor so saying "MAGE IS THE BESTERIST BECAUSE IT HAS LAST PRISM WHICH OWNS!!!!!!!!' isn't a fair assessment.
Melee
Melee is defined by its astonishingly broad amount of attack types while retaining high defense.
+ Yo-yos bridge the gap between "helpless" and "usable" for early progression
+ Powerful close-range options start to provide limited mid and long-range fighting capabilities as the game progresses
+ Later game options like Flairon allow for a powerful mixture of long-range attacks while retaining stellar defense
+ Fetid Baghnakhs are frighteningly powerful with the correct build and can rival Lunar-tier equipment in the right settings (multiplayer)
+ High defense builds are nigh unkillable
- Fairly boring and straightforward until Hardmode happens
- Yo-yos bridge the range gap until later options are available, but require accessory investment and NPC luck to truly shine
- High Defense builds can't really abuse the sheer power certain weapons offer
- In multiplayer, raw DPS builds can't upgrade from Beetle to Solar without interfering with tank builds unless one player wants to sacrifice an accessory slot for Flesh Knuckle (tank, Crimson) or Putrid Scent (DPS, Corruption)
Ranger
A sheer damage build, rangers are defined by their simplistic weapons having variety in use by switching up the ammunition they use.
+ Powerful and straightforward, no-nonsense raw attacker class
+ Access to Holy Arrows on shotbows allows for astronomically high amounts of damage
+ Like, really, these guys put out single-target damage that's off the charts
+ Set for life if Titanium spawns in the world due to having an arguably more powerful set bonus for their playstyle than Hallowed or Shroomite
+ Easily hits 100% critical hit rate in endgame without giving up any damage
- Weapons don't really change much; all that upgrades do is make you fire more shots that are more powerful
- Reliant on ammunition effects to really bring home the bacon; infinite ammo pouches are convenient but the lack of secondary effects is horrifically noticeable
- Some ammunition types require grinding
- Shroomite Armor can be a hassle if you use all three types of headgears and its stealth effect is nearly unusable in singleplayer
- They aren't really unique in using guns or bows since Summoners tend to use their weapons as a sidearm as well
- Requires a lot of emblem grinding
Mage
A class that has a lot of identity problems due to the clunky Mana system acting more as just a magic-specific ammunition bar. Potentially powerful crowd control options, but middling single-target damage and doesn't really bloom until later in. Great in multiplayer.
+ Gets two different potions that are specific to their own damage type for ease of use and damage output
+ Mana Regeneration Band and Mana Regeneration Potion combined allow for nearly indefinite casting
+ Razorblade Typhoon and Razorpine are incredible weapons for crowd control and single-target damage, respectively
+ The only multiplayer healer in vanilla thanks to Spectre lifesteal set healing nearby players
- Doesn't really do any damage until Spectre and Fishron/Invasion-tier gear
- Playing as a healer is a severe damage cut
- Relies on a Corruption world for Band of Starpower
- Uses a LOT of Deathweed and Fallen Stars
- High reliance on potions for basic usability lends itself to teaching players to use Mana Flower and deal with Mana Sickness
- Mana Sickness is stupid
- Like, really stupid
- A ranger runs out of ammo and just fires musket balls, a mage runs out of mana and refills back to like 250 and takes a stacking 20% damage cut that hits upwards of 50%
- Seriously, Mana Sickness is stupid
Summoner
A curious mid-range fighter that can put out astonishing amounts of damage in the right hands.
+ Summons do a lot of interesting things and don't cycle out too quickly so you get time to appreciate them
+ Lots of playstyle flexibility if you use other weapons alongside your summons
+ Dungeon Defenders crossover content allows for a wide variety of armor mixing and matching, such as gaining a ranged damage bonus along with a minion count bonus
+ Safest class for building; if you just want a character to build and mine on they're easy to gear up
- Is basically married to multiple accessories in order to get the most amount of minions possible
- Getting maximum damage on your minions is a strange process that can seem intimidating
- Certain minions do piercing damage, limiting the types of weapons and ammunition you can use
- Can, and will, accidentally spawn Wall of Flesh on a regular basis
