Z series if you want widescreen. That's it's only benefit, other than an optional titanium lid.
R series if you're on a budget.
T series if you want a 4:3 thin & light.
Personally, I prefer the T series to every other laptop I've used before. I've not messed with a Z series, admittedly, but I didn't care for the options that were being allowed with it at the time. R series has a plastic lid, and generally feels more mainstream. I'm hesitant to say Dell-like...but it feels more plasticy and unnecessarily bloated due to its use of plastics in the places that a T would use magnesium, or an X titanium.
If I were personally given those choices, assuming the T was a 14.1" form factor and SXGA, I'd take it over any of the others.