I'd have to go with RB1, granted - by today's standards its graphics are crap, and so is the sound... but the involvement it had was something else.
You could "paint" your plane using a pre-set of colored parts, I had a Nieuport 17 (my favorite plane) with a blue upper wing, white lower wing and grey fuselage. When you became an Ace, I think, you could have your own personal aircraft- so at any point in the war you could have any aircraft thats in service. In 1918 I was fighting Fokker DVIIs and DVIIIs (the mono-wing parasol fighter that came in an expansion pack) with the Nieuport 17. If you dove too fast in some of the planes, like the Nieuport 17 or Albatros series, the wings would break in half because the lower wings were extremely weak and small.
Then there was the things like "An Ace has challenged you to a duel!" during the campaign, and you could fight various aces. The letters would prescribe what altitude you were to be at, and what time... certain aces like Lother von Richtofen would bring 2 wingmen with him.
There was also the feature where you would be transferred to crappier and crappier squadrons if your performance was poor, or you could be invited to some of the more elite/crack squadrons if you did well. Squadrons like R.N.A.S. 8, Escadrille 28 (I think, whichever one had Charles Nungesser in it), and Jasta 11 (Richtofen's squadron, which would later be an integral part in J.G.1- the Flying Circus).
If you were wounded and landed behind friendly lines, you would spend time in a hospital. If you crash landed behind enemy lines, or on the front lines (easily discernible because of the trench warfare style), there was a chance of evading capture.
Basically the whole campaign system was what made Red Baron 1, I don't think any system since then has really accomplished the design that went into that system... then again- only one company has really tried WWI since then, and it was Dynamix again with Red Baron 2/3D... but since Damon Slye was no longer with Dynamix, RB2/3D just didn't have the vision behind it.
Its a good game of course, with probably the best dynamic campaign system- but the things mentioned above were not done as well as RB1... or missing fromit completely.
-SW