Hello HeadTrauma, and welcome back to IL-2!
If the last pack you used was HSFX, than I would say a lot has changed in modpack world since you were last in.
I'll give you an overview of the modpacks:
SAS 5.3 (for IL-2 version 4.12.2) and SAS 6.3 (for IL-2 4.13.3) are the gateways to modding IL-2. If you like to simply use stock IL-2 and add your own choice of mods, SAS 5.3 or 6.3 is the way to go.
Ultrapack uses IL-2 version 4.10, and it is mainly designed and optimized for online missions and campaigns. This is what the F.A.C. online group uses.
HSFX was for 4.12.2 last time I used it, and it is optimized for offline game play. It is said that HSFX still has the most realistic battles and flight models, but I am not an authority on HSFX since I too have not used it in quite some time.
CUP (for 4.12.2) was until recently the most complete modpack. It has been superseded by BAT, so I would not recommend you use it now due to this fact and also because CUP is much more difficult to install compared to BAT. BAT includes all of CUP and more.
BAT (for 4.12.2) is the most complete modpack in the history of flight sims to date. It includes nearly every aircraft and map that is made available on this site, as well as hundreds of ships, vehicles, and an insanely massive object pack which takes about 15 minutes to scroll through. BAT is made up of 4 included era-based packs: Dawn Of Flight (WWI), The Golden Age (Interwar years, Spanish Civil War, 1930's military aircraft), World At War (WWII and 1946 aircraft), and The Jet Age (Korean and Vietnam wars, international jet aircraft, and a few modern jets such as the F/A-18, F-16, and Harrier). Altogether there are over 1,000 aircraft and 600 maps in BAT. Other things like extra ships and vehicles correspond to their respective era packs. Since BAT has pretty much every significant mod ever made for IL-2, in theory, any campaign or mission made in an earlier version should work in BAT. BAT is the only modpack (as far as I know) that includes the "Command and Control" mod, which is a collection of triggers, special effects, and randomization objects. Whether you want a complete array of planes, or you just want to try out a plane you've never flown before, BAT is the most complete option. Can't help posting some screenshots here:
VP Modpack is the newest addition to the mod world. Rather than implementing BAT's mindset of including all the available aircraft, vehicles, and maps, VP includes only the highest quality content. Though VP has far less content than BAT, VP makes up for it in detail. BAT is simply beyond the scope of VP in content. The aircraft default skins have been beautifully re-worked by VPMedia, and almost all the default and even some mod maps have been re-textured with much more detail. Quite a few of the maps in VP have "atmospheric" effects, in other words, dirt airfields instead of the lame stock rectangle ones. If you haven't seen these yet, you must at least give it a try!
So many screenshots can not even display the awesomeness of BAT and VP Modpack!
I hope this helps you get an idea of what these modpacks can do! As for which one you should use, that is up to you depending on what is most important to you.
-Captain Dawson