One nice thing about the solo expansion to Conflict of Heroes is that you can play a solo firefight in an hour or two. Well, I can now, anyway. My first game did take me a couple of days, as I was still learning the rules, especially the AI rules. (And I spent a day before that just reading the rules.) But now I whiz through scenarios with few if any rule lookups. I make a move, turn a card to check if my guy is Spent, turn a card to see what the AI does, turn a card to see if the AI guy is Spent, rinse and repeat until I lose. :)
I own EA Ardennes but haven't played it yet. It looks great, but that's a big rulebook! I do enjoy D-Day at Omaha Beach, but it takes a lot more time than CoH/solo. Ditto with RAF and Fields of Fire.
Right now I'd say COH/solo is my favorite solo wargame. That said, I wish it had more scenarios in the box; it ships with only 10 scenarios, and I'll have to buy the Firefight Generator if I want more. Fortunately, a new solo CoH module is supposed to be released in a couple of months; I'm not sure whether it will pertain to the Guadalcanal game or the Kursk game. As I mentioned earlier, the downside of CoH is that you have to buy and learn the base game as well as the solo expansion, so there's a significant investment of cash and time up front before it starts to pay off.