The results are fairly interesting. It isn't surprising that the Gulf War (the first Iraq war) is the top choice. That was the "good" Iraq war. Or that Afghanistan does pretty well - 9/11 was staged from there and AQ was strong there, after all.
For my money, though, it's ISIS or none of the above. To me ISIS is the only "enemy" that is both pretty darn evil AND a non-trivial threat. We didn't really "need" to attack any of those other folks. Some believe the humanitarian grounds were sufficient in the Kosovo mess, and maybe that's true. Certainly there were atrocities and stopping them was a good cause, but the conduct of that war makes us look less like the good guys than I usually like when we are riding to the rescue.