The 'Best' speakers - as expensive as you can afford. More importantly you will be limited to choice by the size of the originals.
Ideally a four set system for the front, mid range in the doors with separate tweeters either on the dash or fitted into it.
Mid range or lower frequency units in the rear doors, and then assess your need for a boombox in the boot.
Before you do any real work, investigate the mains cable and speaker wire routing into the boot, pretty sure the fuel tank is between the back seat and forward end of the boot, so be careful with drills. This also means that sound from the sub will be severely compromised by the amount of material between the boot and passenger compartment.