I think a lot of it has to do with preference. There are many brands to choose from. It also depends on how much water you want to push and the dimensions of your 84 gallon?
First off how much current are you looking for, strong or weak. Either or I would recommend Aqua Clears line of powerheads. The 70 or 110 would probably do the trick for ya. I own a few various sizes of this brand and they seem to do a great job.
I bought 2 340gph LifeTechs off ebay for under 30bucks a few weeks ago...maybe not the greatest brand out there but my experience with them so far they are high quality fully submergable pumps. They do their job very very well. I mean their function is just to move water not do tricks, if your looking for just a good solid pump Id recomend them