I bought upper-level speakers based on size (fitment) and price (not too cheap). Premium speakers are $400 a pair and there is a big drop in price to the next level at <$180 per pair.
The front speakers are component type (separate tweeter): JL Audio C2650, $160 approx
The rear speakers are 2-way: Alpine SPR-60, $110 approx
Both are 6 1/2" size.
Here is my post. Any speaker is better than stock. The head unit (for 4 speaker systems) is made by Sony and can output about 20W/channel max. so you will get 15 W of good sound. Don't buy high power speakers that need more power to drive them.