Splits are the go, for sure. Everyone's made some good suggestions here, but it depends on a few main things:
* Are you gonna run them straight off the head unit? If you have a high power unit you should be able to get away with this, but look for a speaker with high sensitivity (i.e. good efficiency so that the speaker runs loudly AND efficiently from the small amount of power a head unit pushes out, compared to the amount an amp puts out). Remember that the power ratings on a deck are almost always measured in PMPO (Peak Music Power Output - the amount of power that is able to be pushed out in small bursts, and usually with at least some distortion) which is far less accurate than an amp's power output when measured in RMS (Root Mean Square - the amount of power that is CONSTANTLY being produced with minimal distortion). If you're using an amp to run your speakers, then you can sacrifice efficiency a bit to get better sound and build quality in exchange.
If you're gonna be running your speakers directly off the deck at high volume levels, there's a good chance that they'll be distorting and will therefore be damaged a lot more easily.
* The most important thing: do the speakers you sample sound good to YOU? Everyone's ears and tastes are different, so only YOU can decide if YOU like the way a speaker sounds. (When you hear some speakers you like, but are still shopping around, get EXACT model numbers so you can compare apples with apples.) After all, they're going in YOUR car, and primarily for YOUR enjoyment. If you like the sound of the speaker(s) you've sampled, then you can decide on the next point.
* How much do you wanna spend? Ryan suggested some nice gear, but if you're on a budget these may not suit you (then again they might - some people are lucky enough to be able to spend $250,000 on a pair of speakers for their living room, along with their $25,000 CD player, $30,000 amplifier/receiver and another $30,000 on their plasma TV or projector and screen. I wish I had these options!!!). You never know, you might end up being happy with a pair of ReSponse speakers from JayCar.
Set yourself a budget and work around that.
MB Quart's and Boston's are nice, I have never heard Dynaudio's so I can't comment. Focals: a lot of specialist home and studio speaker manufacturers use Focal drivers in their gear, so that should give you some indication on how highly their products are regarded...