A real shared world
The point of an idle MMO is other players: a market where prices mean something, bosses fought together, and guilds. IdleWorlds has all three, plus live chat.
Progression that lasts
Look for goals that take weeks, not an afternoon. IdleWorlds has 34 zones, 10 skills, raid gear from 5 bosses, set bonuses and gems.
Easy to start, easy to keep
No download, no account to start, progress while offline, and a phone-friendly interface.
What you get in IdleWorlds
10 skills: Combat, Mining, Smithing, Herbing, Alchemy, Jewelcrafting, Spellcrafting, Tailoring, Woodcutting and Construction. Each one feeds the others, so a sword starts as ore you mined yourself.
34 zones to fight through, world bosses the whole server hits together, and 5 raid bosses for guild groups of up to 8.
A player-run market with listings and buy orders, guilds of up to 20 players, live chat, leaderboards, and a solo self-found (SSF) league if you'd rather do everything yourself.
Your character keeps working while you're away, so you come back to finished crafts, levels and loot.
Questions
Which idle MMO is best?
It depends on what you enjoy. If you want skilling with a real player economy in the browser, try IdleWorlds: it takes seconds to start.