TLDR: Jackbot (1 Star) > Jackbot (0 Star) > Fursus (1 Star) > Mr. Autofire (1 Star) > Armorita (1 Star)
Succeeding in the game requires a lot of grinding - Worlds (for XP, Coins, Equipment and Upgrade material), Castles (for Shards, Hero Medals, Rings, Crystals, Equipment), Daily Events (for Hero Medals, Shards, Crystals on weekdays; Coins on the weekend), and Tournament (for XP and rewards if you can get high enough) and Hidden Treasures in all of them (for Coins, Energy, Upgrade material and Crystals).
A lot of this grinding will be in Worlds, because it is only constrained by Energy (Castles are additionally limited by Keys, Daily Events by runs per day, and Tournament has a cooling down period (per hero) when you don’t complete the run) and it is the best way to get XP (you can get more XP/level at higher tournament floors, but you need to have a really strong loadout to make use of that). You can get more loot (at the cost of a little XP and more play time) using a Farming hero with the 1 Star Equipment Drop Chance skill (i.e. one of Mr. Autofire, Armorita, Fursus or Jackbot) because it improves your chances of getting equipment (weapons, armour, gear) as loot by ~50%.
The baseline option is Mr. AutoFire because everyone has him, and you can get him to 1 Star just from the free shard on Monday, and the 5% All-Hero dodge chance when you get him to 20 is quite handy too.
If you have Jackbot (either from the BattlePass or subsequently from a banner summon gate) he is the best choice because of the Jackbot passive (see how to maximise drops) even if you don’t have him at 1 Star (and currently the only way to get Shards is from Summon gates (he drops in regular summons if you have him unlocked, but only in Banner Summons if you don’t), which is a bummer)
If you don’t have Jackbot, Fursus is a very useful farmer (and the only one with good Attack) although it will usually take a while to unlock him and star him up (either by farming shards in the Ice Castle or getting lucky on the Summon gate); the 10 shards in August’s Battlepass helped speed this up.
Unlocking Armorita and starring her up would be slightly faster than Fursus (because there’s a 50% chance of shards in the metal castle being Armorita and some of the one-time rewards have her shards) but IMO you would be better off just using Mr. Autofire (especially when you’d almost certainly get him to 1 Star earlier than you could with Armorita and you’ve already invested in levelling him up in the early days when he was your only hero)