Without an auxillary amplifier, many aftermarket speakers will sound like crap because they require more power to drive them properly than the factory head unit is capable of providing. Most aftermarket speakers require more power to make them sound good so when you turn up the factory head unit louder which isn't usually powerful enough to put out much more power, it often just starts distorting if you're not careful which aftermarket speaker you install.
What you want to do is select the speaker with the highest "Sensitivity" rating which will be expressed in dB (decibels). A speaker with a 91 dB Sensitivity rating will sound noticably louder at the same power from the amplifier than a speaker with an 88 dB rating will. Conversely, a speaker with a 91 dB Sensitivity rating, for example, can get long with an amplifier at roughly half of the power than a speaker with the reduced 88 (again for example) dB Sensitivity rating will.
So with a stock radio that won't produce a lot of power, I'd be FAR happier with a moderately priced speaker with a 90 dB Sensitivity rating than an uber-expensive high-end speaker with only an 87 dB (for example) sensitivity rating. Generally speaking and everything else being nearly the same, the speaker with a higher sensitivity rating will sound better than a speaker with a lower sensitivity when talking stock power OE amplifiers or radios.
Put another way... a speaker with a 91 dB Sensitivity rating being driven with a 15 RMS watt amplifier will sound about the same as a speaker with an 88 dB Sensitivity rating being driven by a 30 RMS watt amplifier... everything else being equal.