RTCW was good, but ET is what made the game memorable. Even years and years after the free ET release, you could still find 64 player servers packed with a waiting list to get in.
Almost all modern MP shooters owe their design to ET; afaik it was the first to have accumulated experience over the course of a campaign (3+ maps). With the huge number of players that loved it, it was only natural most shooters that followed had some sort of experience mechanic.
Another thing that made ET great, was the ease of making maps and playing them. You just downloaded user made maps in seconds before the map started, which meant you as a map designer could simply upload your map to your server, and everybody could play it - a design feature sorely lacking in modern shooters (but probably because there would be no need for map pack DLC if thousands of user made maps were out there). I remember playing on a giant pool table, where the balls moved (and you could use them for cover).
Splash Damage (the ET makers) haven't come close to making something that good again, which is a shame. ETQW was fun (on the PC, not the console), and I played with some of the big names from ID in the alpha/beta which was cool.