And as a *pure* rogue class, Arcane Trickster is the only choice I would consider. Especially if you make good use of the invisible and permanent Mage Hand, like dropping stuff to the ground, then throwing it with the mage hand, thus de facto giving you more actions per turn, AT is actually perfectly solid.
From a powergaming perspective, both Thief and Assassin are VERY front loaded and only interesting for a 3-5 level dip. Thief naturally combines super well with Open Hand Monk, Assassin naturally combines super well with Gloomstalker Ranger. From a powergaming perspective, pure class Thief and Assassin are among the weakest subclasses in the game, because Rogue got nerfed compared to the original D&D5 definition.
Pure Thief is okay. Extra bonus action for messing around with Cunning Actions is nice. It also gives you some insurance in case you miss your standard action attack, you get up to 2 more tries to get your Sneak Attack damage in for that round. Among other things you can utilize extra bonus actions for (Such as using off hand crossbow shots to break bottles of water you've placed down next to enemies to make them Wet for your casters to blast them apart with Lightning/Cold spells)
If you're not powergaming and using maxed builds and/or abusing multicast Haste... Rogue is perfectly fine. Sneak Attacks do scale pretty well and they're ridiculously easy to set up in 5e (And nothing is immune to them. Freaking Undead and Oozes...)
Rogue only really falls off when powergaming because Haste is so overtuned with Larian's homebrew that getting 3 attacks out of a Fighter (Or Warlock multi) or an additional spell out of any caster is insane... While Rogue doesn't get to Sneak Attack more than once per round...
Assassin is the awkward subclass. Since you have to metagame pretty hard to actually get any use out of its meagre bonuses (Since you have to know where the fights are so you can prepare to sneak up on them. Then of course figure out how to deal with the plethora of forced dialogues that ignore not being visible or the forced fights that occur with or without dialogues)
Still, you're a Rogue so you have your Sneak Attacks but outside that... Yikes. Assassin really ranks up there along with Illusionist and Enchanter for contending for "Worst subclass in the game". It's only real merit is that people actually do use the subclass when multiclassing with Gloomstalker, while both Illusionist and Enchanter simply don't have any merits for builds, be they pure or multiclassed (Especially when compared to the powerhouse Wizard subclasses like Abjurer, Evoker and Diviner)
