Lets start off with a somewhat controversial opinion, the stock intake manifold (upper and lower) is the best flowing RB20 intake manifold period. Yea I get its ugly, but it is has the best performance. The main element that matters for an RB20 is the intake runner length. The longer the runner the more low to mid range torque the engine has. The shorter the runner the less low to mid range torque. These engines have a reputation for being gutless down low so they need all the help they can get. If you look at the engine flow numbers above, the intake manifold is not what is preventing RB20s from making more power, it is the flow of the engine itself. An aftermarket intake manifold is not needed to increase your RB20 power potential. The engineers at Nissan had infinity more resources than anyone who has made an aftermarket solution and I trust them.

That being said, I understand it is ugly. If I were use an aftermarket intake manifold, the Plazmaman upper intake would be the best performance choice. It keeps the long stock bottom runners and moves the throttle body to the side so a more efficient intercooler can be used. I personally would avoid a "Cut and Shut" manifold and a cast or billet "Greddy style" intake manifold. A "Cut and Shut" where an OEM manifold is modified to have a forward facing throttle body. This lets you change the position of the intercooler piping, but there have been many instances of people claiming air not flowing equally into all the cylinders and could cause a few cylinders to lean out. The "Greddy Style" manifolds use very short runners and make the engine extremely lack luster at low rpm. That style intake manifold paired with a larger turbo will produce a lot of turbo lag. Also to note is the throttle body diameter. Running a significantly larger throttle body may cause more lag or a lazy response.

Ill skip all the gibber gabber, go and watch this video on Youtube and it should answer 90% of your questions:

Click Here For Exhaust Manifold Design Video

Due to the large number of available exhaust manifolds for RB20s, I will let you pick what is best for your application. 

The stock log manifold is actually one of the best designs for a quick spool. It has very good turbo response and there are many bolt on T3 turbos that can make pretty much any power level you want. The longer the runners the more potential for turbo lag, but the freer flowing and potential to make more peak hp. The long runner GTSR equal length style will produce more peak power (plus look and sound like heaven on earth) while a shorter runner style like the R31 Autech will have better turbo response. 

A properly designed tubular manifold will take advantage of exhaust scavenging and your engine just feel like a weight has been lifted off its shoulders. I would recommend staying T3 turbo flange. I will get more into that with the turbo choices, but remember this is a 2.0L 6 cylinder and may struggle to spool up a large T4 flanged turbo. 

Top mount, mid mount, or bottom mount? Personally I think the bottom mount manifolds look amazing and make a great sound, but again the choice is up to you. Its hard to pick a bad exhaust manifold, just keep in mind this is a small displacement engine so huge diameter and long runners have the potential to create more turbo lag.