Chethan refers to custom fields, not custom objects.
The answer is certainly not to use the API, since it does not correctly distinguish between standard and custom fields, and does not show hidden fields.
The most inclusive list of standard fields that I know of is:
Company Info / Account Owner Email Address
Company Info / Account Owner First Name
Company Info / Account Owner Last Name
Company Info / Annual Revenue
Company Info / Billing Address
Company Info / Billing City
Company Info / Billing Country
Company Info / Billing Postal Code
Company Info / Billing State
Company Info / Company
Company Info / Company Notes
Company Info / Company SFDC Type
Company Info / External Company Id
Company Info / Industry
Company Info / Main Phone
Company Info / Marketo Data.com Id
Company Info / Marketo Data.com Status
Company Info / Num Employees
Company Info / Parent Company Name
Company Info / SFDC Account Num
Company Info / SFDC Type
Company Info / SIC Code
Company Info / Site
Company Info / Website
Lead Info / Acquisition Date
Lead Info / Acquisition Program Name
Lead Info / Address
Lead Info / Anonymous IP
Lead Info / Black Listed
Lead Info / Black Listed Cause
Lead Info / City
Lead Info / Cookies
Lead Info / Country
Lead Info / Created
Lead Info / Date of Birth
Lead Info / Department
Lead Info / Do Not Call
Lead Info / Do Not Call Cause
Lead Info / Do Not Call Reason
Lead Info / Email
Lead Info / Email Invalid
Lead Info / Email Invalid Cause
Lead Info / Email Suspended
Lead Info / Email Suspended At
Lead Info / Email Suspended Cause
Lead Info / External Sales Person Id
Lead Info / Fax
Lead Info / First
Lead Info / Full Name
Lead Info / Inferred City
Lead Info / Inferred Company
Lead Info / Inferred Country
Lead Info / Inferred Metropolitan Area
Lead Info / Inferred Phone Area Code
Lead Info / Inferred Postal Code
Lead Info / Inferred State Region
Lead Info / Is Anonymous
Lead Info / Is Customer
Lead Info / Is Partner
Lead Info / Job Title
Lead Info / Last
Lead Info / Lead Notes
Lead Info / Lead Owner Email Address
Lead Info / Lead Owner First Name
Lead Info / Lead Owner Job Title
Lead Info / Lead Owner Last Name
Lead Info / Lead Owner Phone Number
Lead Info / Lead Partition
Lead Info / Lead Revenue Cycle Model
Lead Info / Lead Revenue Stage
Lead Info / Marketing Suspended
Lead Info / Marketing Suspended Cause
Lead Info / Marketo Data.com Id
Lead Info / Marketo Data.com Status
Lead Info / Marketo Unique Code
Lead Info / Marketo Unique Name
Lead Info / Middle
Lead Info / Mobile
Lead Info / Original Referrer
Lead Info / Original Search Engine
Lead Info / Original Search Phrase
Lead Info / Original Source Info
Lead Info / Original Source Type
Lead Info / Person Notes
Lead Info / Phone
Lead Info / Postal Code
Lead Info / Priority
Lead Info / Rating
Lead Info / Registration Source Info
Lead Info / Registration Source Type
Lead Info / Relative Score
Lead Info / Relative Urgency
Lead Info / Role
Lead Info / SFDC Created Date
Lead Info / SFDC Id
Lead Info / SFDC Is Deleted
Lead Info / SFDC Type
Lead Info / Salutation
Lead Info / Score
Lead Info / Source
Lead Info / State
Lead Info / Status
Lead Info / Unsubscribed
Lead Info / Unsubscribed Cause
Lead Info / Unsubscribed Reason
Lead Info / Updated
Lead Info / Urgency
Opportunity Info / Amount
Opportunity Info / Close Date
Opportunity Info / Description
Opportunity Info / Expected Revenue
Opportunity Info / Fiscal
Opportunity Info / Fiscal Quarter
Opportunity Info / Fiscal Year
Opportunity Info / Forecast Category
Opportunity Info / Forecast Category Name
Opportunity Info / Has Opportunity Line Item
Opportunity Info / Is Closed
Opportunity Info / Is Won
Opportunity Info / Last Activity Date
Opportunity Info / Lead Source
Opportunity Info / Name
Opportunity Info / Next Step
Opportunity Info / Probability
Opportunity Info / Quantity
Opportunity Info / SFDC Created Date
Opportunity Info / Stage
Opportunity Info / Type
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