In my opinion the best proof is communication or chat logs. You need cohabitation proof and lots of financial support proof too, but communication proof is the MOST important.
Think about it, 2 friends can live together on the same lease. 2 friends can maybe show financial support and shared bills, or even photos together! 2 friends can get friends and family to write letters. Chat logs prove you are in a relationship if you talk about that you love each other, and if you say anything sexual it helps prove you have supported your partner economically, socially, emotionally and physically which is what they look at. Chat logs can also mention that you are "coming home from work" or things like that which show you were living together. Chat logs really show the immigration officer what each of you were thinking and when and that is really the best proof for them.