It depends on what your goal is. But, let's back up a second. You applied to ENT and since they are hard to get, you should wait and see what day they give you. If you applied for a specific date, odds are much lower that you'll get it. Once you get the anchor ad, apply to other advertisers on the specific day the anchor gives you, be that BookBub, ENT, FKB&T, or OHFB.
That said, back to the goal. If your goal is simply to make the most money you can from the promo, go with every free advertiser you can and a few $5 ads. As has been said, OHFB costs more than either ENT or FKB&T, but you'll sell more with OHFB. So, if your goal is to sell as many as possible, to gain the highest rank possible for visibility, OHFB should be your anchor, of those you're considering.
However, BookBub is king. You won't sell as many, nor make as much profit from any other advertiser.