IMO, Zero was the pinnacle of the series. That game had it all. The atmosphere, the billions of cars, the absolutely incredible Techno-Thrash soundtrack, dear god. The physics are a little...
funny and you have to exploit the AI to win anything, but it is SO good. TRX3's music is really bad (at least, what I've heard of the OST), has way less cars since Genki couldn't do the code-name workaround, and overall just doesn't have the atmosphere of TRX0, not to mention it has only one traffic car thanks to Japanese automotive companies' hatred of street racing. Drift 2 is hard as balls personally and I don't find touge racing to be as fun as highway racing.
Zero is just something else. I'm not sure how to describe it. It's got that amazing Japanese flair and the atmosphere is dark and foreboding, yet also mesmerizing. Flying down the highways of Tokyo at 190+MPH with a rival hot on my tail while
Doom Breaker blasts out of my speaker is one of the most unforgettable experiences of my gaming career. The OST for this game is just incredible. The music when you're just cruising and looking for rivals is amazing, as is the music when you're in the menus. Japanese devs know how to make godlike OST's.
My only real criticism of Zero is that it pretty much forces you to use very few cars. You can't re-challenge rivals for money, and upgrades are so ludicrously expensive (and necessary since it always feels like your rivals have 300 more HP than you ever do) that you pretty much can only drive one car at a time. The C-Class cars are borderline worthless since they'll get outclassed really quick, so the only sane way to start is to buy a B-class car, then grab an A-Class car and use that the whole rest of the game. I love my R33 but I really wish I could have a few more cars just for the variety.
If they're making a new TRX, then I hope to god that
a.) It's a spiritual successor to TRX0
b.) It's licensed and translated by something who wasn't half-braindead like TRX0 was
c.) It has some old-school Skylines! The R-series are great, of course, but damn I want to drive one of those Hakosukas!