To me it seems like the difficulty is skewed towards the book end campaigns. By far the easiest has been Red Lightning '79 since the Soviet missiles/aircraft hadn't caught up to NATO quite yet so if you fly the F-15/16 campaign it's pure domination (I tried a MiG-23MLA campaign using the editor, I can get past like, two missions MAX). Once you play the '86 campaigns though and you have to deal with -29's, -27's, and -31's, it gets real hard. The Mig-17's in the '56 and '62 campaigns will eat just about anything alive, they are so far away better than anything else in the sky.
I recommend a multi-role aircraft, just to keep things fresh with mission choices. Also, dive into (but backup!) your campaign files and try swapping out aircraft, replacing older mods with newer ones, changing the skins, etc. It's cool to use Volker's F-4E's and have them based at Hahn and Spangdalhelm and they all have the proper unit markings. Also, if you have SF2:Israel, download the USAF/NATO F-16A (TW) mod, the Mirage Factory F-16A is pretty old. Make sure you bounce any changes in the Campaign_Data.ini with the Campiagn.ini file as well.