MoO 2 was great and generally an improvement over the original; never tried the 3rd since EVERYONE said it sucked. You can get it off of Abandonia, and it runs fine with DOSBox.
Red Alert 2 was also pretty good, 3 was a sell-out failure. Generals has absolutely nothing to do with Command and Conquer, but (with the expansion) was a pretty good game in its own right.
I like SupCom: Forged Alliance a lot, and it does help that it still has an active modding community, but SupCom 2 was also a sell-out failure that shares nothing with its predecessor.
Far Cry 2 has nothing to do with Far Cry, but I like it for what it is even if it's not a game you can sit down and play straight through (has a few infuriating flaws).
L4D2 is a solid improvement over the original. The only thing I really don't like about it is that double pistols really suck. A Magnum is always better and I would rather each loadout have its advantages and disadvantages (like they did with the main weapons). The melee weapons are all samey too, but if all 10 or so of them were different that might be too much.
Mercenaries was fun, but Mercs 2 was just stupid. It's too bad that the original is practically unplayable on 360 (freezes all the time), not that that would matter to you if you're PC-only.
Halo 3 was the game Halo 2 was meant to be, and (given my experience with the beta) I'd say Reach will also be good. The abilities, different health system, new game types, reworked weapons and balancing, and improved map editor should bring some freshness to the tired franchise. Oh, and don't buy ODST at full price, it's just a map pack and short campaign with a game mode you'll soon get tired of.
Mass Effect 2 fixed almost everything that was wrong with the first, and brought a totally different style of gameplay too. I liked it; the exact same mechanics as the first game would have been dull.
