Hearts of Iron 2 + expansions (available now as a complete version for around $20 is various digital download locations) is a great pick and one of the most detailed simulations you will see. It is far more detailed than its cousin EU:3 and follows the "Victoria" school of thought that Paradox seems to be deviating from (luckily expansions are fixing that). Hopefully HoI3 (coming sometime within a year or so) will be as complex.
You have to credit Erik for his honesty, he could have easily tried to "sell" you on one of those two games, but did the right thing (he believes in customer satisfaction rather than the hard sale, a very rare and commendable trait). Both of those two games are equal to SC2 in abstraction and complexity. I am a big fan of SC2, but I can see why some might object. The expansions for SC2 added a lot of great details, and the new Pacific Theater release is quite nice!
Now GG:WaW is an interesting bird. It is fairly abstracted, but also fairly complex in other areas. It plays much like Axis and Allies with a bit more complexity (unit interactions, rail movement, espionage, supply, and infrastructure management). But it still has a very boardgame feel to it.
World in Flames is going to be a hugely detailed game. I am leery of it a bit, as I was disappointed in the terrible UI and poor feedback of Empire in Arms (also developed from a boardgame equally as complex as World in Flames). If the UI is workable and the game is made so that people who have not played the WiF boardgame canb easily learn the system (tooltips people, tooltips - is it that hard), then WiF is likely to be great fun. But the above posters are right, the release might be a ways off (although it was supposed to be close to release around now, before it got pushed more and more back - another flashback of my anticipation for Empire in Arms days).
EDIT: Re:Joe D - GG:AWD added a nice espionage system, an number of decision events, several tweaks to combat (and some additions to units and sea units). I like both, and I would say that AWD brings some very good things to the table. I suggest checking out the features list to see the changes.
< Message edited by Son_of_Montfort -- 1/29/2009 11:54:51 PM >
"Neca eos omnes. Deus suos agnoscet!"
(Kill them all. God will know his own.)
-- Arnaud-Armaury, the Albigensian Crusade