A few things, not trying to be rude but:
Rabia is a lady, not a dude. She doesn't do anything of note other than manipulate Cadance and Radiant Hope. Granted, she was the only Umbrum not to be defeated by the combined forces of Rarity's parents, a few hamsters and the Cutie Mark Crusaders throwing apples, so that's...something? But she was effortlessly taken out by Sombra. Siege of the Crystal Empire was stupid.
Also. Celestia and Luna were both stated to have infinite power and were able to be capable of reshaping the entire multiverse, which is infinite in scale, and Discords mind alone is the size of infinite realms, which he and Accord could both reshape. Sombra is probably stronger than them since in the alternate timeline, Applejack remarked that even with Celestia, they couldn't stop Sombra.
Gusty was explicitly implied to be nowhere as powerful as Grogar, who himself has no actual feats, as she had to defeat him by stealing his bell. Theirs nothing that puts her in the same league as even, say, Twilight Sparkle.
A proper tier list would probably be:
Bookworm. Was shown to be able to destroy entire universes.
Starswirl the Bearded. Generally in the same power range as the Princesses, but expressed they were far and above superior than him, even when they were just teenagers.
Pony of Shadows. Was sealed away by Eris and not to much more powerful than basic Stygian or Starswirl iirc.
Princess Eris. Was actively scared of Luna and not to much more powerful than unicorns like Tempest Shadow. Kind of a pushover in her storyline.
Princess Cadance. Probably in the same league as the other princesses, generally treated as such at least.
Queen Chrysalis. Shown to be able to fight Luna, but was defeated after a beam struggle in Siege of the Crystal Empire.
Princess Luna / Princess Argent / Dupluna. Reasons above.
Princess Celestia / Princess Solar / Nightmare Star / Fauxlestia. Shown to be equal or a bit stronger than Luna.
(it was implied during Cosmos that the Abyssinian Royal Family, King Aspen and Queen Novo may be in the same league as Celestia and Luna and have their own versions of the Elements of Harmony but otherwise have no real feats)
Sombra. Reasons above.Consistently shown that Luna and Celestia can't really defeat him without artifacts
Cosmos. Took all the monarchs to defeat, but was beaten by Discord.
Discord / Accord. Has yet to have any real opposition and has cosmic tier feats as stated above which by themselves are higher than other characters.
Tirek at full power. Has the power of Discord and the Princesses. Normal Tirek is hard to place.
Other characters of note would be Empress Twilight Sparkle (siad to be the most dangerous force Equestria ever encountered), Bad Apple (had some of Luna's energy), King Anubis (implied to be the Nightmare Moon of Anugypt), King Leo (Celestia's cousin) and Storm King.