Lego set for 17 year old
You're probably the only one who can answer that question, but here are a few thoughts anyway.

If you're interested in mechanic complexity and how machines work, Technic is indeed a good way to go. And as you hint, it can evolve into robotics, whether with a NXT or with anything else you fancy.

If you're after building complexity, I wouldn't recommend the architecture sets. The few I build weren't very challenging, and I find their price a bit on the heavy side. The bigger ones (21005 - Fallingwater or 21010 - Robie House may be more challenging, though.

However, sets with a high number of parts will usually mean the build is more complex. For example, 10214 - Tower Bridge has 4287 parts, and if you compare its price with the 2276-parts Architecture Robie House, you'll find it much more interesting.

Similarly, the various modular buildings also have high parts counts and are fun to build and display afterwards; plus, they go well together when you get the chance to have more than one.

As an aside, notice the "display" aspect I mention is important too - what do you want to do with the set you'll end up buying? If you're after replay value (creating new things constantly, the Creator theme has great sets with good play value, which are reasonably priced.

On the other hand, if you're concerned about your set keeping some value, or if you're a fan, license sets may be a good choice, especially Star Wars which also has some very big and complex sets.

Last, keep in mind LEGO also give age ranges for its sets. They're estimates, but a set marked 16+ is likely to be more challenging to you, so you'll want to look for these. You can easily browse the sets by age on the online LEGO shop, and even set limits for price and number of parts.

In any case, whatever you end up buying, don't hesitate to come back and share your own experience.