I'll just give you some of my personal experience since I have changed a ton of speakers lately. You can see the list of cars I have or did have so I'll tell you what worked for me and what really didn't.
First, if you want the best sound for a speaker, JBLs are it. That's just my personal opinion, but they are fantastic speakers. I bought my '94 Mark VIII Lincoln with premium sound and they had JBLs. So when I bought my '97 Jeep and I needed to change the speakers I went with JBLs. They sounded fantastic in the Lincoln and Jeep. This is the exact speakers I had:
http://www.amazon.com/JBL-GTO638-6-...d=1404474877&sr=8-1&keywords=jbl+car+speakers
My brother in law now runs those in his '94 Chevy Silverado as well and he loves them. They are a bit pricier. My younger brother has a pair of Sony Xplods in the '88 F250 that we own and I will say this, they sound pretty good for the price, but they also don't have a great sound range. Now, he does only have two in the truck and you will obviously have more in the explorer, but they don't push as much sound as some speakers. These are the speakers my brother has in the '88 F-250:
http://www.amazon.com/Sony-XS-R1642...5062&sr=1-31&keywords=sony+xplod+car+speakers
I also tried kicker speakers a few times and I will say they have good bass and they'll hit pretty good for regular car speakers, but they don't have a full range of sound and the actual speaker part leaves a little to be desired. If you love a ton of bass, they certainly work well. I put a pair of those JBL that I linked above in the front of a 2002 explorer with kicker speakers in the back doors and the combination was solid. The JBL put out a lot of sound and they have good bass as well and then the kickers add even more bass. Don't get me wrong, the kickers aren't bad speakers at all. I was happy with the ones I had, but I also like a lot of bass usually.
Rockford Fosgate is a lot like Kicker to me as well. A lot of good bass and not as much "speaker," but still pretty good speakers.
http://www.amazon.com/Rockford-Fosg...&qid=1404475429&sr=1-20&keywords=car+speakers That's the link to the fosgates I have been around. Again you'll find that they do bass better than a lot of places, but the speaker component could use a little work. I would say these and the kickers are pretty equal.
There's also JL Audio but it's going to be very high priced. I think for the price, JBL is a better option. I've not heard too many good things about Pyle speakers, but I can't say I've personally been around them. My brother in law worked at a car shop in high school and college and he says they're not the best, though still better than 90% of stock speakers. You could throw a pair of those in the front doors and something just a touch pricier in the back doors and still have a nice system.
I know a lot of people like to have all of the same speakers, but it sometimes works out better or at least OK to match your own up as well.
Last but not least, my brother in law has been dealing with sound for over 10 years professionally and every time I ask him about head units or speakers, he always goes back to Kenwood. One of the best bangs for your buck, Kenwoods are pretty nice when you consider everything.
http://www.amazon.com/Kenwood-KFC16...1077068&newCar=1&s=car&vehicleType=automotive Obviously you'll need to find the right size, but those are similar to what you would want to look for in Kenwoods.
So a quick recap:
JBL - the best in my opinion but a little pricier
JL - Great but too pricey for me
Kenwood - most likely the best for what you're looking for, but that's my opinion and everyone differs in what they look for in a car speaker.
Kicker - Fine speakers but more bass
Rockford Fosgate - basically the same as kickers in a lot of ways
Sony Xplods - Not a bad cheaper option, don't put out as much sound as I would prefer