This question got me thinking. I have several webers, but the one I would pick is one I don't even own. It would have to be the Kamado. Seems like the one that really can do it all and do it well.
Gas assist Performer, preferably a restored SSP. Best single grill any person could own. All the versatility of a normal kettle with the gas start and extra table space. All for somewhere between $200 for a used or restored model to less than $500 for a brand new model.
E6! Of the many Webers I have burned out or owned I love my E6. The reason is it is fuel frugal and can do anything the other Webers can do with less work. The Weber R&D on these is pretty complete. My previous favorite was the 26"kettle. I liked the WSM but got tired of cleaning the water pan, I found out after I sold it many use the water pan empty with good results.
I guess for me it would be the grill I've been cooking on for the last 24 years, a Genesis 1000. I'm familiar with it in any weather condition at this point and I don't need any additional thermometer or probe to tell me when my food is done. It does everything I need it to do and has never left me wanting more from it.
The question doesn't specify "type". But, for me it would be a "true" Genesis. Best an older x000 model second best, B or C. Otherwise no use for any other except maybe a kettle.
Probably a Performer, something I don't have now. If my two kettles and WSM suddenly disappeared I'd buy a Performer. But I'd probably buy another WSM within a month or two. (Yes, I know it said only one, but I hereby invoke the Kobayashi Maru rule.)
I would say due to the six month charcoal ban in the summer I would have to go with our E320 NG. It also doubles with being our outdoor oven in the summer. With it being NG it is always ready to go, no gas bottles to mess with.
Yes, I'll agree with the other Genesis owners -- my 12-yr-old E-330 is the most versatile and convenient of all I've used. It will do all a gas grill is supposed to as well as low and slow (brisket and pork butt). You can add smoke flavor (though not truly smoke as in smoke ring) or a rotisserie. With my added upper grate it doubles as an oven including producing enough heat for stoneless pizza. I've used other older Genesis models finding them to be completely comparable. Adding the 14.5 GSM makes a perfect compliment, but the Genesis is my first choice.
Well, six months ago I would have said the Performer Deluxe, hands down, no question about it. But now that I have a Kamado Joe Classic II, the Weber I would keep now is my gasser.