Starcraft 1 factions are well done: music, gameplay, art.. the only bad thing is the lore/story, i share similar view with the autist RaggleFraggle in this thread:
https://rpgcodex.net/forums/threads/starcraft.139970/page-4
Protoss are boring 'good wise guys' space elves.
Zerg are just bugs slaves of Kerrigan.
Terran is a bit better, but we only follow a group of characters, not much about their civilization or factions in there.
I may be asking too much for a RTS game, but this sucks. Keep in mind i've only played Starcraft 1, the second game ruined the atmosphere, and fuck that Reynor/Kerrigan romance plot ruined the already mediocre story. And i don't read wiki for lore, i'm not this autist.
Lore/story aside, everything is great, it's the only RTS i've played so far in each faction feels unique, to some extent(more on this later). The music in all three of them is amazing and distinct:
Warcraft 3 tried the same but just ripped off some ideas from starcraft, the Necromancer terrain thing is ripped off from Zerg, the nightelves movable buildings is ripped off from Terran, the story itself is ripped off from Starcraft, Arthas is basically Kerrigan, the music is major decline from Warcraft 2:
Youtube seems to agree with me, Warcraft 2 ost having double the views despite its age, the only memorable track in Warcraft 3 is this one:
Of course i haven't played all RTS to judge the factions, but from the ones i've played:
Age of Empires 2: 3/10 for the factions, there's only one unique unit to each of them, gameplay is mostly similar, native americans can not use horses and trebuchet, and have unique warriors, i think that's the biggest difference, no unique music. Haven't played the campaign.
Age of Mythology: 7/10 for the factions. factions have different units, gods, ways to gain favor, abilities, but they lack unique music and atmosphere, only the intro music is unique. Still great. Campaign is not separated for each faction, you need to play the entire campaign in order first, so that is a negative point.
Dune 2000: music are the same, few unique units, gameplay is the same, but the factions fmvs are great so i give you that, 5/10 for the factions. House of Ordos fmvs specially good:
https://youtu.be/BZrStHhSL1w
Starcraft: unique music and atmosphere, unique units, unique gameplay mechanics and campaigns, this is the best i've played yet, 9/10 for the factions.
Total Annihilation Kingdoms: i do remember the factions being well made, the cutscenes as well for the campaign, however it has been a long time since i played it it's hard to judge. The factions have unique units, music, hero and mechanics, i think one of the factions was pretty good on water but shitty on land. The expansion supposed to add another faction but i haven't played, anyway i give it a 7/10. Campaign is not separated. This is judging the factions only, the game itself is flawed as hell, some missions are terribly designed, but the factions are well done.
Warcraft 2: great music, art and atmosphere between the two factions, gameplay is mostly the same however, and the campaign as well. 6/10.
Warcraft 3: unique music, atmosphere, great art, some variety in the gameplay but not as much as Starcraft. 8/10.
I've played other RTS games, but not enough to judge. Going back to Starcraft, while it does have variety, the resource collecting of each faction still mostly the same, and the base building as well, save for some small differences. I think that's the biggest problem with RTS, resource collecting and base building are too static and defensive, they always give you a base and enough resource to collect for 30min right in the beginning of each map, this could be different, it should give a few military units so you explore and search for resources and where to build you first base, sorta like Civilization, and the resources should not last too long or there are unique resources that gives major advantage, so you should be fighting for resources right from the get go, turtling should be discouraged unless you already conquered good spot for resources, and maybe each faction having their own type of resources as well. This is some ideas which may not work well for multiplayer, but fun for single.
For the poll, it was a draw between Terran and Zerg, but in the end i choose the Zerg, it's the most fun to play, i like 'evil' vibes from it as well, the last mission of Brood War, surrounded by tough enemies and having to rush them(you can not turtle in this one)while listening to this:
took 10 hours, losing over and over again until i succeed, best experience i had in a RTS.
But i haven't played the other games, except for Warcraft 2/3, so i'm only choosing between these three games.