This is a long debated question for "electric" drummers-- I can't even count the amount of threads over at Vdrums.com on this topic. I'll just give my experiences:
--Mackie SRM450's-- some say they lack thump. I think if your playing a big room-- agreed. but for small shows/personal monitor/practicing/ or at "home" use-- I love 'em. I have a pair of them now as well.
--I use the Bose PAS for some stuff-- basically "like" a PA System very loud, decent bottom end, great top end, and very compact/versatile considering it's "like" a PA (4 inputs, some different sound shapes for the inputs...) Drawback-- very pricey...
Roland TDA 700's- They are big. But they have wheels. They sound really, really good for electronic drums. But I use a pair of them. One is Great for a stage monitor or for practice/home use.. but for playing out you would need more/pair... again, pricey- and in-fact, I think they are discontinued.
-- ISP Tecnolgies Tripower 15 -- basically it's a PA speaker that's Powered. WOW. that's all I can say --Price -wise I think it is very comparable to anything of it's make.
I've heard nothing but good things about the db Tech. 405's-- but have no personal experience with them.
I think alot of it has to do with "what" you are using them for, for the majority of your time. Your style of music comes into play, how loud you need to be/etc.. there's so many variables-- do a search on Vdrums and you'll have a few hours of reading for yourself...