Old School Runescape - Jagex has been known to crack down on bots pretty hard. There's always plentiful amounts of bots though. If you do multiple accounts at once, you'll need proxies.
Team Fortress 2 - Aim botting. I find it fun. Others may not. You'll collect currency to trade while you play. You can use sandboxie with multiple accounts and idle instead if the currency is all you want, You'll have to get through the pain in the ass Steam Guard that doesn't let you trade for 7 days until you use the Authenticator for that amount of time and continue to use it. You'll also have to buy something from the Mann. Co Store so your accounts are not F2P.
WoW Private Servers - Namely, Vanilla. You'll need proxies for multiple accounts for this as well. Be sure to only use Private Profiles. Blizzard tends to kill off any major Retail bot. Ban waves have been very frequent since WoD. Unless you really know what you're doing, gold farm, and sell to make a profit before a ban wave happens. Retail isn't worth it anymore imo.
Guild Wars 2, Black Desert Online, Elder Scrolls Online may be worth looking into.
Diablo 2 is likely always an option.
Runescape is probably the easiest game to bot.