I've heard great things about a lot of the intercoolers currently available. If you intend to stay stock turbo, it's hard to go wrong and you have a few options. For the highest of quality I've heard amazing things about the CP-e Delta core; which is what I will likely be going for. The construction on that thing is top notch and from what I can find in data, it doesn't disappoint in perform either. It is the only factory fitting intercooler I have seen data for that actually DROPS charge temps during a pull with minimal loss in pressure. It is built extremely well. CP-e has a huge reputation for quality. The downside? It cost $700 and is very heavy.
Alternative to that is Deadhook Motorsports. They have no shortage of fans and their products speak for themselves. They work well and Russ has a lot of testimonials behind him to back his product up. I had originally thought I had some data that suggest his factory fit intercooler did not flow very much better than stock, but it turned out to be erroneous. The price is right for his factory fit and it performs well.
MAPerformance recently came out with an intercooler as well. I haven't seen any performance numbers but it looks to be built well and is competitively priced. I'm very eager to see how this one performs and it has my attention as one to watch.
I also hear great things about the AirTec intercooler. If you look around on Google, there was a guy named Razorlab who did a LOT of extensive testing for it on and off the track. Of all of the available intercoolers out there, i think this one probably has THE most data collected and compiled by a user in the community. It performs very well, is constructed well, and has a reasonable price.
A lot of people run the Mishimoto j-line too. It's very good at cooling and is huge with some modification of the front assembly needed I believe in some cases. From what I have seen this one does not flow as well as other intercoolers. That's to be expected however as it is not an intercooler designed for our car directly. It's inexpensive and cools well, though. I believe Mishimoto is designing an intercooler that is for our car, but I haven't heard much about it lately.
Cobb is... well, it's Cobb. People don't seem to have much opinion on it. Best I can gather is that it works adequately and is built well. For the price you can probably do better, though. Of all of the intercoolers I've seen data for this one has eluded me.
I've heard that of all of the available intercoolers out there the mountune probably performs the worst by virtue of being tube and fin in design. Everything else is bar and plate. I've seen SOME data that backs this up, but not enough to make a definitive call on it.
I only really know about factory fit intercoolers though. People rave about the DHM Race Intercooler and rightly so. If you're willing to replace the crash bar and have big turbo dreams, it's probably the way to go.
Here is the data I managed to compile on intercoolers that I could find.
http://datazap.me/u/commonplatypus/stock-vs-dhm-vs-cpe-vs-airtec-vs-jline?log=0&data=
You can toggle the different plot lines for them, although I would take the CP-e with a grain of salt. It flows at about 22 lbs/min on mass airflow in other datalogs I have managed to see. A bit above these in this compilation, but not far outside of the range of error of margin for climate, air quality, barometric pressure, etc. As far as I know, these were all on the Cobb OTS Stage 1 tune with on charge pipe upgrades (which do not benefit us at stock turbo size anyways).
Hope this helps!