1.6 is still legal in the US. However, local codes in areas like California are more likely to require 1.28. Looking at Home Depot inventory however shows that 1.6 is going extinct. Not very many 1.6 options any more.
Pretty much anything you buy now will have good flush performance. Just stay away from the economy models and you should be fine. You really can flush a bucket of golf balls with these new toilets.
I would second the Toto recommendation just because the mechanism is probably more durable. With cheaper units you will find that after 4-5 years there will be issues with the mechanism. Having to replace the entire flush valve assembly is not like it was in the old days. Each brand has proprietary components that will be expensive and hard to get.
The hardest part is picking the options. Short vs tall, round vs elongated, anti-microbial coatings, one piece vs two piece, etc...
My favorite feature is the tank mounting point that is an inch or so above the top of the bowl. Such a simple thing but it makes all the difference in keeping everything clean.
