I really, really liked Halo. It's probably my favorite single-player FPS. I realize that it probably can't hold a candle to Half-Life 2 or Portal, which are far stronger in terms of plot and construction. But you know what? When I play an FPS, I usually don't care about plot. Halo offered me incredibly fun, immersive gameplay that I've never found in any other PC FPS. For me, Half-Life 2's loading screens and huge landscapes where you simply played with physics kind of ruined the gameplay experience for me. Portal was entertaining in more ways than one and very well-made, but there were times in the game where I would have to slowly plod from one room to another only to discover that I needed a cube, then backtrack over the same route to find the cube, and retrace my steps AGAIN when I discovered that I had incorrectly placed a Portal. Then I started noclipping to get from room to room faster, but then I would pass into the next room and spoil the puzzle for myself... I did enjoy Portal a lot, and if I hadn't been driven to cheat out of boredom, I probably would have enjoyed it even more. But the fact that I was forced to cheat out of boredom ruined it for me. Halo didn't really have anything to disrupt gameplay. The game is pretty linear, meaning that you don't spend a lot of time wandering empty corridors trying to find your next objective (something that I HATE in FPS games), and the weapon system was great. In Half-Life 2, there would be times when I'd be frantically scrolling through my list of ten weapons trying to find my shotgun as fast zombies bore down on me, but Halo's simple weapon system never caused me any grief. I actually liked the fact that you were only allowed to have two weapons and sometimes have to drop your sniper rifle to pick up a rocket launcher, or discard a really good weapon because your weapons were redundant and weren't effective against a variety of enemies. To be honest, I never really played it a lot online and I felt that the online play was rather repetitive and boring. Halo is my favorite single player FPS of all time. It's only $10 on Amazon, I highly recommend that you pick it up. Unfortunately, because of some deal that Bungie signed with Microsoft when Halo was released for Xbox, Halo 2 is Vista only and requires that you install and run Windows Live. There are hacks to make it playable under Windows XP, but getting past one of the levels requires that you actually interrupt gameplay to tweak the game settings if you're running XP. Given that Halo 2 for PC is significantly more expensive than Halo 2 for Xbox and that an Xbox can be bought for cheaper than Vista, it seems like a bad investment. I didn't buy Halo 2 even though I own a legal copy of Vista; I didn't want to feed Microsoft's greed (or go to the trouble of installing Vista).
As for multiplayer, I played Counter-Strike: Source a lot about a year ago and I like it better than CS 1.6. This may be because I'm a graphics whore, but to be honest 1.6 didn't seem significantly better in terms of gameplay. All the differences are in the physics engine. All the guns are exactly the same (although they perform differently under different engines), but I actually liked Source better for gameplay because the shotgun and p90 seemed better in my opinion, which gave the weapon assortment more variety. Also, I hate having to buy ammo separately from my gun. I know, it adds an additional element of strategy and sometimes when you're rushing you don't WANT to have any clips in reserve, but I'm very impatient and having one more thing to re-buy at the start of every round is not something that makes me happy. I think that 1.6 is considered more "pro" since hitboxes are smaller so that you can't clear rooms of enemies by simply spraying your automatic rifle at your opponents' feet, but I never really felt like Source was too luck-based. Also, I did not like the shield from the original Counter-Strike AT ALL.
I stopped playing CS:S when Orange Box game out. Team Fortress 2 is probably the best multiplayer game I have ever played, period. One thing I've found is that a lot of FPS games, striving to be realistic, sacrifice gameplay. The thing is, I don't like my FPS games to be realistic: I don't like it when there are useless weapons that are simply inferior versions of better weapons, and I don't like it when I can at any time instantly be killed by a single grenade, rocket, or sniper rifle bullet. Team Fortress 2 is horribly unrealistic, both graphically and in terms of physics. The graphics look like something modeled after Pixar and the cast of characters is incredibly funny; the game is absolutely rife with voice clips which you can activate by taunting or calling to your teammates, but a lot of them trigger automatically and they're usually quite appropriate. I also like the class system a lot. There is no one class that can dominate in any given situation, and the game constantly shifts around the map so that one class can't rule the entire field. The classes are all so different and each of them has very specific strengths and weaknesses, and the game actually forces you to play so that you promote your strengths and compensate your weaknesses. In Counter-Strike, an automatic rifle should be able to outgun an SMG at close range, but if the SMG user has better aim and faster reflexes, he can headshot the rifleman before he has a chance to respond. In Team Fortress 2, if you're playing as a scout and run across a Heavy, you cannot win simply by aiming for the head and pulling your trigger. You have to pick with your pistol from a distance, jump in and fire scattergun shots when he doesn't have his minigun spinning, or simply run off to complete the objective and leave your team to deal with him. In Team Fortress 2, the player with the best aim does not always win; winning is usually decided by which team has better teamwork. A soldier and a sniper with excellent aim will be overwhelmed by a soldier and medic that work co-operatively. I've found that a lot of other games suffer from balance issues in which a lot of weapons are redundant; in Counter-Strike you have five different SMGs, only one of which is actually good for competitive play (and another which is fun to use but a bit expensive). Also, some weapons are clearly superior to others; a $4750 sniper rifle is going to be better than a $1500 smg. In Team Fotress 2, weapons all have their uses; it would be difficult to call a sniper rifle better than the minigun; the sniper rifle, when fully charged, can kill anything with a headshot but has a low rate of fire making it difficult to take on large numbers of enemies, whereas the minigun has a high rate of fire and no reload and tears everything to ribbons at close range. Also consider that the sniper rifle is wielded by the sniper who has only 125 HP compared to the minigun-wielding heavy's 300 HP, but doesn't suffer from an incredibly large hitbox or unbearably slow running speed
Another thing that Team Fortress got right is gametype: the best maps are offense versus defense, where the red's only objective is to defend until time runs out, and the blus only have to capture within the limit to win. This promotes the Team aspect of the game more than Capture the flag games, where you have people squabbling because not everyone is working towards the same goal ("stop defending the flag and help me cap!," "stop attacking and come back and defend the flag!"). This is one of the things that made Counter-Strike great (Terrorists attack, Counter-Terrorists defend), but Counter-Strike matches often ended with one team winning by simply killing all of the opposing players, rather than accomplishing their objective. Team Fortress 2's game mechanics do an excellent job of promoting team play. Team Fortress 2 has been sapping my life for the last six months; it's pretty much the only game that I play. Team Fortress 2 can be bought for $20 on Steam or $18 on Amazon (you'll still have to register it on Steam to play). Alternatively, it can be bought as part of the Orange Box (which also includes Portal and the three Half-Life 2 games) for $40 on Steam or (currently) $30 on Amazon. I'd recommend that those who don't have Half-Life 2 already get the Orange Box, since it allows you to play third-party Source mods. There are too many to name, but I will say that SourceForts and Zombie Master are worth the download (although they both tend to get repetitive, depends on who you play with).