What do you mean by 'most stable' ? In which way https://github.com/odoo/odoo/tree/master can be more stable than https://github.com/odoo/odoo/tree/8.0 ?

I imagine he means which of the saas versions are the most stable: 1-5

Then probably https://github.com/odoo/odoo/tree/master shouldn't be there. I think that both the question and the currently chosen answer are a little confusing.

most stable mean, very less bugs, most bug fixed and merged.

well most patches and merges does not mean most stable. However maybe the question is why do you want to use a saas version? It is not supported by odoo and there is no upgrade path from odoo nor the community.

@stephen, then which version to use for my cloud server ? Why not to use for self hosting ? if those branch works great then what is problem ? How you suggest best SAAS version to use ?

I think the confusion is in the name. SaaS (Software as a Service) implies cloud deployment however the SaaS versions are not odoo's cloud version and v8 is odoos self-hosting version. SaaS-5 should be considered odoo v7.4 without the official minor version support (upgrades). odoo v7 and SaaS-1 is the same software and odoo v8 and SaaS-6 will too be the same software. There is no magic put into the SaaS version to make it cloud ready. SaaS is just odoo's internal version that they use on their hosting option. If you ever want to upgrade odoo in the future you should install version 7 or version 8 when it comes out in a few weeks. Check out this thread: https://www.odoo.com/forum/help-1/question/odoo-v8-vs-saas-6-vs-ocb-v8-whats-the-difference-58606

Do you know which branch is going to be the saas-6 that you referred to? If somebody installs from https://github.com/odoo/odoo/tree/8.0 as it is now, do you think that there will be problems upgrading to the stable odoo version 8.0?

Honestly I would expect it to be a fork of v8rc1 the 8.0 tree. The master branch is already starting the v9 development. Can you upgrade without problems? Maybe but I would not count on it.