I have a wall I need to insulate and have only 2 inches to work with, so I am planning to use some kind of insulated panel board and want to maximize the insulation. Cost is no object, so if it is very expensive, that is ok. What is the best possible option? Note that I do need something that is commonly commercially available quickly, so I can't use some custom solution like aerogel or anything like that.
Why is aerogel not an option? Would it not arrive soon enough? http://www.buyaerogel.com/product/thermal-wrap-8-mm/
I believe polyisocyanurate (R-5.6 per inch) is correct for pre-made foam boards per Ecnerwal's great answer but you'll want to make sure to silicone caulk around the edges to ensure zero air penetration between the board and the frame it's in.
You said money is no object so if you're not opposed to spray foam then I suggest closed cell as it is R-7 per inch and it will act as a vapor barrier. Hire a contractor and get it done in less than 15 minutes.
I've used spray foam kits to seal my rim joists but it was honestly a hassle. If you're not used to the motion then it ends up uneven.
Open cell is more readily available but at R-3.8 per inch it would be less desirable.