Co-founder is the correct answer for your question. But even if you call them founders, it wouldn't be a big deal.
"co-founder" implies more than 1 founder. But, "founder" may be used in the single or multiple senses.
I've recently seen a disturbing trend where people seem to be using both of these terms at one company, granting "founder" status to a few and lower "co-founder" status to others. As far as I can tell this originated at Facebook where Mark Zuckerberg wanted to distinguish himself from all others that claimed/litigated status as founders. I also have seen/heard of this when someone is granted founder status later in the formation of a company and hence only considered a co-founder vs. a founder.
In general, to me - if there's multiple founders, all are co-founders, if there is only one then one is a founder. Certainly one can refer to all co-founders as founders, but I find the dual-class structure distasteful and disingenuous.
I've also seen examples where "Founder" is used to denote the person who worked on the idea prior to bringing in the other "co-Founders." This was the inspiration for my title ("Founder and President"), although my partners certainly have the right to use the titles "co-Founder" or even "Founder."
Why not approach this question from this standpoint - if company is founded by one person - he is a founder. If company was started by many of them - call them co-founders. In the latter case no one feels discriminated or offended. Make a situation more compromise-wisely and dimplomatic (in case of many (co)-founders).
I have the same issue. Although I don't see it as a big deal, I believe it's important to find a way to name them properly.
We are a team of 5 people with equal shares at our company. I've came up with the idea and the other members in our team got involved to put it into practice. We made a video presenting our idea and initially I thought it would be fine to name me a founder and name them co-founders, but now I'm not completely satisfied of that.
I think that since we have equal shares in the company, we all should be considered founders and have different functions assigned to distinguish ourselves. I will be using what "nyc_ceo" suggested and put Founder/Function to each one of us.