stock front speakers are not bad
For my 2011 xB, my priority was to get good sound yet not spend too much money. So I kept the stock base Pioneer unit and the front OEM speakers. The front OEM door speakers, while not the clearest at producing treble sounds, have pretty decent bass. At least the tweaters are mounted up high in the front doors, which is good.
The rear door speakers are mounted too low and the sound they put out is muddy. Thus they were the prime target for improvement.
I replaced the rear door speakers with Kicker DS65.2 component speakers. These are 2-way speakers with a separate tweater and crossover. These speakers are rated at 90 dB efficiency which is better than many others. Also for component speakers they are not too expensive.
I mounted the 6.5" woofers on homemade 3/4" thick MDF spacers to move the speaker inwards towards the stock speaker grills. I also put foam weatherstrip all around the speaker rim to seal it to the door panel as much as possible.
I mounted the tweaters up high and forward in the rear doors to improve the soundstage, and used the included angle mounts to point the tweaters slightly rearwards.
I mounted the crossovers on the doors and under the door panel, and cut a small hole in the forward part of each door panel to access the 3-position switch (0bB, +3dB, +6dB) after installation. Currently I am running at 0dB.
I am happy with the results. The bass is getting better as the speakers wear in, so I feel I don't need a subwoofer. The soundstage is now in the middle of the car (where my ears are) and the highs are much improved.
While I won't rattle my doors off or impress the neighboring cars at stoplights, my tunes now sound much better than the stock setup. The Kickers were only $78 (shipped) so I feel I have good value for what I spent.
Best of all I impressed the most important person: me!

But that's just my opinion, your needs and tastes will vary.