For a SaaS app with login, signup, MFA, and enterprise SSO, the best default choice is usually:

Recommended setup

Use a hosted authentication provider that supports:

• Email/password + magic links
• MFA / step-up auth
• Social logins
• Enterprise SSO via SAML and/or OIDC
• User management + orgs/teams
• Session management
• SCIM if you need automated provisioning later

Good options

If you want the shortest path to production:

1. Auth0 / Okta Customer Identity

   • Strong enterprise SSO support
   • Mature MFA and org features
   • Good if enterprise sales is a core part of the business
   • Can get expensive at scale

2. Clerk

   • Very developer-friendly
   • Great UX for signup/login/session handling
   • Supports MFA and organizations
   • Enterprise SSO support is improving, but check whether it covers your exact SAML/SCIM needs

3. WorkOS + your own auth

   • Best if you want to build your own user/password auth but outsource enterprise SSO
   • WorkOS is especially good for SAML, SCIM, directory sync, enterprise connection management
   • Common choice for B2B SaaS

4. Firebase Auth / AWS Cognito / Supabase Auth

   • Cheaper and simpler for basic auth
   • Usually less ideal for polished enterprise SSO/MFA/org workflows
   • Better if you’re earlier stage and don’t need heavy enterprise features yet

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My practical recommendation

If you’re building B2B SaaS and expect enterprise customers:

Use:

• Clerk or Auth0 for primary auth
• WorkOS for enterprise SSO + SCIM

This combo is common because:

• You get a good consumer-style auth UX
• You offload complex enterprise identity requirements
• You can add SSO later without redesigning the whole auth stack

If enterprise SSO is a must-have from day 1:

Use Auth0 or Okta Customer Identity

• They handle the broadest set of enterprise identity needs out of the box
• Better when you want one vendor for everything
• More setup and usually higher cost

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What I would avoid

I would generally avoid rolling your own auth unless:

• You have a very specific compliance/security requirement
• You already have strong identity expertise in-house
• You’re okay spending time on edge cases like password resets, MFA flows, account recovery, session security, and SSO protocol handling

Auth is one of those areas where “simple” becomes “security-critical” very quickly.

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Suggested architecture

A solid SaaS auth model is:

• Users
• Organizations / Workspaces
• Memberships with roles
• Identity methods
  • email/password
  • social login
  • MFA factors
  • enterprise identity connection
• Sessions
• Audit logs
• SCIM provisioning for enterprise plans

This lets you support:

• Individual users
• Team accounts
• Enterprise orgs with SSO
• Role-based access control

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Quick decision rule

Choose based on your primary need:

• Fastest implementation + polished UX: Clerk
• Most enterprise-ready all-in-one: Auth0 / Okta
• Best for B2B SaaS with enterprise SSO add-on: WorkOS + your auth layer
• Lowest cost but more DIY: Cognito / Supabase / Firebase

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If you want, I can also recommend a setup based on:

1. your frontend/backend stack,
2. whether you’re B2B or B2C, and
3. whether you need SAML, SCIM, or just “Sign in with Google/Microsoft.”