What is the best platform to create a member based CMS website? (e.g. Squarespace)

Wordpress is a great platform and you can add a plugin to take care of the membership aspects. But it requires configuration, updates etc.

You could for a all-in-one solution such as the Ringgle platform (http://www.ringgle.com) which does not need any backend configuration, tinkering or development etc.

Drupal is our suggestion for this because of a number of factors: 1) It is built on a community focus, that is, with multiple levels of permission for user roles, 2) it is able to be extended to have many different types of content, e.g. a single "paid user" might be able to create: blogs, images, coupons, or the like 3) it is highly supported by a wide developer community, 4) it has many different member solutions built-in such as e-commerce, marketplace functionality, private messaging, flagging, five-star rating, wishlists, and groups to handle members-only content

I've produced multiple member sites on Drupal and written about it here: http://10kwebdesign.com/10kblog/step-step-setting-membership-community-site-using-drupal -- Drupal is currently moving into version 8 so now is a great time to start planning what you want the membership community to do and what kind of features you want to allow.

It is according to your budget. If you want something quick, easy, and economical wordpress is what I would use and have on several of my sites. Once you get up and running and making money you may be able to make adjustments. One of my motos is to never small decisions stop your momentum. If using the very best results in little or no movement, it is probably best to leave the best for last. Keep Moving!
Michael Irvin
michaelirvin.net

There are many Website Builder Platforms. It's an entire industry that seems to be thriving - the top 5 players having all raised millions in funding this year (2014).

This list is in no particular order. Some of them offer the ability to create membership type websites (but not in the standard way where your members would access a CMS - that can only be done custom).

Website Builder Platforms

BaseKit Based Platform (HostGator, SiteJam, etc) - http://www.basekit.com/
GoDaddy Website Builder - https://www.godaddy.com/
Jimdo - http://www.jimdo.com/
Populr - https://www.populr.me/
Readymag - https://readymag.com/
SquareSpace - http://www.squarespace.com/
Voog - http://voog.com/ (edicy back in the day)
Webflow - https://webflow.com/
Webnode - http://www.webnode.com/
Weebly - http://www.weebly.com/
Wix - http://www.wix.com/
Yola - https://www.yola.com/
Zoho Sites - https://www.zoho.com/sites/
Moonfruit - http://moonfruit.com/
Biznessweb - http://biznessweb.com/
Aircus - http://www.aircus.com/
imcreator - http://www.imcreator.com/
Cloudcannon - http://cloudcannon.com
Webydo - http://webydo.com
Sidengo - https://sidengo.com/
Homestead - http://www.homestead.com/
Virb - http://virb.com/
Breezi - http://breezi.com/
Strikingly - http://www.strikingly.com/
uCoz - http://www.ucoz.com/
Flazio - http://www.flazio.com/
Jigsy - http://jigsy.com/
Webstarts - http://www.webstarts.com/
Webs - http://www.webs.com/
Vistaprint - http://vistaprint.com/
Simplesite - http://www.simplesite.com/
SnapPages - http://snappages.com/
Exai - http://www.exai.com/
Site Sumo - http://sitesumo.com/
DoddleKit - http://www.doodlekit.com/
Pikock - http://www.pikock.com/
Gutensite - http://www.gutensite.com/
Flavors Me - http://flavors.me/
Airsquare - http://www.airsquare.com/
Mobsyte - http://www.mobsyte.com/
Pagekit - http://www.pagekit.com/
Goodsie - http://goodsie.com/
Freshcreator - http://www.freshcreator.com/
Oxxy - https://www.oxxy.com/
Cubender - http://www.cubender.com/
Lifeyo - http://www.lifeyo.com/
Froont - http://froont.com/
Mozello - http://www.mozello.com/
Pixelhub - http://pixelhub.me/
Dudamobile - http://www.dudamobile.com/
Unbounce - http://unbounce.com/
Nubook - http://nubook.com/
Zyro - http://zyro.com/
Justaddcontent - http://justaddcontent.com/
Mpress - http://mpress.me/
Livecontrol - https://www.livecontrol.net/
Rocketspark - http://www.rocketspark.com/
The Grid - https://thegrid.io/
Website - http://www.website.com/
Sitezulu - http://www.sitezulu.com/
Sitedity - http://www.sitedity.com/
Nikktto - https://nikktto.com
Website builder - http://www.website-builder.com/
Mrsite - http://mrsite.com/
Websitebuild - http://www.websitebuild.net/
Landerapp - http://landerapp.com/
Simplybuilt - https://www.simplybuilt.com
Instapage - http://www.instapage.com/
Broadsheet - http://broadsheet.io/

I'm sure there's more website builder platforms out there that are not in my list.

I would also recommend Drupal if you want to build your own website from scratch with unique business.

Advantage with Drupal is that is can be completely customisable and experienced Drupal developer can build MVP of products that compare to Squarespace, AirBnb, LinkedIn etc.