I'm of the opinion that every band should have a set of QSC K12s. If you can afford them, get them, whether new or used. K12s are the standard and it's proven by the fact that whenever the subject of powered speakers comes up almost all other speakers suggested are compared in some way to K12s. Our band's rhythm guitarist picked up a set of well used K12s last year and they really are amazing both in sound and reliability. They've been our front of house speakers ever since.
There are other speakers that are also really good and notably cheaper than K12s... EVs, JBLs, Altos, Behringers and others mentioned in this thread. All of them sound pretty good, but none quite as good as K12s.
I myself own a pair of EV ZLX12Ps which our band uses as monitors. They are really, really good PA speakers, notably lighter than the QSCs and because I bought them new, they look a lot better than our K12s. The LCD display is ridiculously easy to use to set them up. We would not hesitate for one moment to use them front of house but we hardly ever do because well... we have the K12s and beat up as they are those still end up in front. The ZLXs are fantastic,
but just not quite as good as the QSCs... neither is anything else.
So again, my suggestion is get a set of K12s if you can in any way afford them. But if not, any of the EVs, JBLs, Altos or Behringers will do. When you finally do upgrade to K12s (and you will...) any of those will make fine monitors