Are you talking hoof boots, or leg boots? I've never used hoof boots. Never felt the need to. I haven't had any horses with abscesed/punctured hooves or anything of the sort that would need extra protection for the sole of the foot.
I do, however, use sports boots. I used to use splint boots alone, but found that even if they were well adjusted, it was hard to find a good brand that didn't slide at least a little bit from where they were initially placed. Plus, they don't really prodive protection for the fetlock and pastern. I like the Professional Choice brand combo boots personally. I only use them on the front, though all four legs just adds more safety. I do reining and my first concern is striking their forelegs with the opposing leg while spinning or doing rollbacks.
Easy care make the best boots I think. They are a little more expensie but will pretty much last you for ever. I have the easy care epics. Those are what I would recomend, and I also recomend getting the gator that attaches to them if you will be doing trail riding.
Easyboot epics!! But make sure you buy the gaiters that go with them! Look on the easyboot website. They are a GREAT investment. I am currently transitioning my gelding to barefoot. He wears the boots on the trail. They will stay on and in place through mud, water, etc. Tip: Drill a small hole in the bottom for water to drain through!!!
I think some people use on all 4 depending on riding. However my both horses hind hoofs are not round (and it's quite common according to the farriers), so I'm not sure how good the boot will feet them.