Currently I am using ClientExec without a complaint. I have used ModernBill and it does have a lot of features, but I call it the carpul tunnel supporter, as it takes 3x as many clicks to get anything done that program. WHMCS seems pretty good too, it has a lot of features, but in the end I went with ClientExec because to me it was easier to use/look at... not cluttery I guess. But it was between those 2 in the end for ease of use, features, and cost! They are both super affordable.