This used to be a pretty popular thread before the hack. Hopefully we can get it going again!
As a rule of thumb, try to keep it short and sweet. A little description/explanation is fine, but please let's not turn this into a conversation thread. TIA
BEST INVESTMENT: NST Short Shift Kit
NST is constantly coming out with great products for our beloved yarii, and the short shift kit is an excellent example of this. It makes the Yaris shift like a more expensive car, and the price definitely won't break the bank. I recommend both stages 1 and 2, but the first stage is more noticeable.
WORST INVESTMENT: 17" Wheels
I don't recommend anything larger than 16" for Mexico roads. Of course this may/may not apply for some people elsewhere, but you should seriously consider a smaller diameter if the roads you frequent are not well maintained.
WORTHY MENTIONS:
NST Pulley kit
The difference is felt instantly. Great price/performance ratio.
1ZZ Throttle Body
Same reasons as above, plus it was a fun DIY project.
UltraRacing 23mm sway bar
Stability has been greatly improved; another "should've come stock" item.