Infinity seems to be the most evolved game of the Trilogy, combining the atmosphere of the first game with the environments of the second, though I have a certain affinity for Durandal's philosophical tirades found in the first (and to a lesser extent the second) Marathon game; these passages are what really seemed to elevate the Trilogy above simply being a computer game series. Infinity does have some interesting terminals, though, especially in the dream levels, and the events in the story somehow seem to have more gravity than those of Marathon 1 and 2.
The levels in Infinity are probably better than those in the earlier games; some such as By Committee stand out in my mind as being particularly good, and I really like the title of "A Converted Church in Venice, Italy". And, of course, the Infinity netmaps are some of the best of any FPS (and definitely the best of any sprite-based shooter), only rivaled by those of Double Aught's "Coriolis Loop" map pack.
Pretty much the only visible problems with Infinity, and these are really just subjective complaints, are that the wall textures lack some of the character of the M2 textures, notably in the Sewage and Water collections; the Infinity ones seem more realistic, and are probably better for most uses, but the M2 ones seemed to give the environments a more alien feel. Also (and this is probably the biggest gripe about Infinity), the game is not nearly as accessible as the first two games; in the earlier Marathon games, even though one might not fully understand all of the nuances of the plot, it was at least possible to tell what was going on the first time through! Of course, this isn't necessarily a bad thing; if Marathon were easier to understand then Hamish would need to find a new hobby, Bungie.org would have seven more gigabytes of server space to fill, and we'd all have to find some way to use all those additional hours of free time. ;)
