We currently recommend https://github.com/WCGH/Waju-Sims/releases as the go-to standalone simulator.
This sim is well-designed and features most serious mechanics in the fight and is completely standalone from the game, meaning it is safe to use without risking your account.
Another interesting sim is the AnoMech plugin. This allows users to play the fight in-game as though they were really there by manipulating the Duty Recorder feature.
Please note that this is a Dalamud plugin that runs inside FFXIV, meaning it is very much against the Square Enix Terms of Service. Do not discuss the plugin via in-game chat.
Warning: originally this was quite unstable, requiring a restart after using it otherwise your game would crash once you got to that mechanic in the real fight. The Github claims to have fixed this, but we have not tested it.