GX, definitely. After playing through GX, going back to the old one felt slow. I know X was 60fps, but it feels like 20fps next to GX (same with Smash Bros. vs. Melee).
I agree with Lizard on the font thing though. Also, I try to read the readout on the performance circle for various machines (the thing that tells you body, acceleration, top speed, and cornering), but it's so small that I can't tell when something's a 3 or a 2.5. In case you're wondering why I care, I was always fascinated by how the different parts change the overall performance, but there's no known formula for calculating what it will be (each custom part might have a different effect I'm thinking, so it's not just weight).
I've always pronounced it F-Zero "X" because I knew there weren't ten titles in the series. But I've seen F-Zero 10 on a couple websites. Conversely, I'm in a habit of saying Mac OS "X" instead of Mac OS 10. Since I usually need to qualify it with what version it is, I say Mac OS X Panther instead of Mac OS 10.3.
