I don't have the budget for every single possible vinegar, (nor every single mustard for that matter!) but I think next time I can afford to I will go for red wine vinegar. I currently have malt, white wine, rice, and balsamic. (And white spirit vinegar for cleaning).
I think I can manage another one in the budget as they are all on rotation and I don't need to buy all at once, but more than 5 seems excessive given that I am on a very tight budget - I think I'll go with red wine vinegar.