FlamingCliffs3 with its simplified F-15C, Su-27/33, Mig-29, Su-25 and A-10A planes would be the most fun to begin with imo. With them you'll still enjoy same advanced aerodynamics model as the full fidelity modules, you'll also get to learn to operate radar, IR sensors and mavericks, learn to dogfight and deal with SAMs. It is plenty enough to learn and not having to worry about complex avionics of full fat modules makes learning the above less of a drudge. My vote is for FlamingCliffs3 and a nice scripted campaign by Baltic Dragon for FC3's F-15C. "F-15 C: The Georgian War" it's is called I seem to remember. It is kinda like Ace Combat with better physics and weapons - lots of acted dialogue, gentle learning curve, good plot and memorable setups with a few branching choices.
Otherwise, if you feel like diving into the deepend, go straight for the full fat A-10C: it's fully complete, next to no bugs and has tonnes of single player content.
Last but not least I'd also recommend getting voice attack, it's immersive and super useful for ordering stuff to AI without using the deep F keys menus.