My usual linter configuration (for browser extensions)

.eslintrc.json

{
    "env": {
        "browser": true,
        "webextensions": true,
        "es6": true
    },
    "extends": "eslint:recommended",
    "rules": {
        // Allow console
        "no-console": "off",

        // Style, indent/whitespace
        "indent": ["warn", 2, {"SwitchCase": 1}],
        "no-trailing-spaces": "warn",
        "eol-last": "warn",
        "no-multiple-empty-lines": "warn",

        // Style, spacing
        "space-unary-ops": ["warn", {"words": true, "nonwords": false}],
        "space-infix-ops": "warn",
        "keyword-spacing": "warn",
        "space-before-blocks": ["warn", "always"],
        "operator-linebreak": "warn",
        "space-before-function-paren": ["warn", "never"],
        "space-in-parens": ["warn", "never"],
        "array-bracket-spacing": ["warn", "never"],
        "no-spaced-func": "warn",
        "key-spacing": ["warn", {"beforeColon": false, "afterColon": true}],
        "quote-props": ["warn", "as-needed"],

        // Style, braces
        "brace-style": ["warn", "1tbs", {"allowSingleLine": true}],
        "curly": ["warn", "multi-line"],

        // Style, misc
        "no-with": "warn",
        "dot-notation": "warn",
        "quotes": ["warn", "double", {"avoidEscape": true}],
        "no-var": "warn",
        "semi": ["warn", "always"]
    }
}

no-var is probably the most controversial.