Enough dilly dallying around. It's time I put some effort into this and finally get at least one S-Rank. I've got a couple games that I think I can achieve this with.
 
Portal
I've gone through the story, beaten the advanced maps. All that's left to really do is find the missing radio signals and then get to work on the challenges. They look tough, but Portal is such an enjoyable experience that I'm looking forward to it. It's been awhile since my brain has had a good workout anyway. 
 
Braid
I just need to do the speedrun. This looks tough, and I normally hate being rushed in games. Still, Braid is an excellent game and with enough practice, I hope I can beat the game in under 45 minutes.
 
Batman: Arkham Asylum
I recently beat this on normal and it was an absolute blast. I'm working on 100% the normal run which could be tough. The combat challenges are very tricky since you really have to maximize how many points on each enemy. The stealth challenges are a bit easier since, for the most part, I can take my time with them. Eventually I'll come back and beat the game on hard. Looking forward to running through such an amazing game again.
 
There it is. Three chances at my first S-Rank. It's going to be tough, but I'd hate to have all these games and not master at least one.

i'd say Portal will be the easiest to get.

Out of those Portal seems like by far the easiest S-Rank to get. Whatever game you go for good luck with it.

Portal the easiest?  I think not.  Golding some of those time trials is an absolutely maddening process, and in my opinion, the least steps challenges are just kind of a joke.  The ergonomics of the game are not what they need to be to make measuring your movement in individual footsteps a reasonable task.  Even carefully taking my time, I often found it not consistent enough about how many steps I was taking to get from point to point even though I had moved pretty much exactly the same way and amount I had the last five times.

Arkham Asylum is by far the easiest out of the three, play it through on hard perhaps, then come back to do the normal challenges and it will feel like a piece of cake. It takes some practice and you'll spend a few hours wanting to rip your own hair out but once you get the knack you'll be fine.

Arkham Asylum wasn't too difficult.  The toughest achievement in that for me was the combat challenges on the higher difficulty levels, specifically Shock and Awe.  You basically have to do the entire level in one fluid combo, and that can prove to be a tad difficult and infuriating when dealing with all the stun rods and guys throwing shit at you.  Eventually, it worked out, though. 
 
I'm assuming you are talking about Portal: Still Alive?  I have not bought that or played it, as I don't know exactly what it is.  Are they the same levels from the original game as well as new levels, or is it kind of its own thing apart from Portal?  If it's the same shit over again, I don't feel like bothering with it.  If you are talking about Portal in terms of "The Orange Box", then good luck.  That fucker is a BEAST to S-rank. 
 
I would also recommend you check out Gunstar Heroes, as that's a pretty easy S-rank for a pretty awesome game.  Castlevania: SOTN isn't too tough either, unless you are no good at running the game with Richter. 
 
Good luck in the endeavors, though. 
 
@JJWeatherman: I don't know why people DON'T beat those guys up anyways!  Maybe I'm just morally fucked in the head or something, but I saw that as a big slap in the face from Joker.  It felt logical to me, and I was rewarded with an achievement for it!  = D

I think the Batman AA S-Rank is relatively easy to get (seeing as how I have it).  The most difficult part of the game for me was using all the moves during one combo. I remember grinding the section right outside where they throw you a party over and over.  Basically I would fly down from the top of the bat cave to where four people are standing around clapping for you.  That was very easy for me to start the combo and I was able to reset the checkpoint when I messed up.
 
I don't remember the challenges being too difficult since I loved those things, so the only other tedious part was grinding/gathering all the Riddles.

Beating the whole game without taking a bullet seems like it would be the hardest achievement in Portal. Out of your list, I've only played Portal. 
 
 
If you want an easy S-rank, I'd say MW2. It's not hard, it just requires dedication. But it's not on your list, so I'd say Portal, I guess...

"Wait, I've been wondering what an S-rank is, and it seems like everyone here somehow knows about it, so can someone tell me what it is? Is it getting all of the acheivments for one game on steam?? cause thats kinda the vibe im getting "

Batman on hard isn't too "hard" really. Especially if you've done all the other stuff before.
 
I was stubborn and had to defeat the big guys (forgot their names) without the batarang ...on hard ...which was ...well it took a few hours :D (I did it in the elevator part where there are about three or four enemy waves coming down the elevator and then one of those big guys ...THAT was hard)

Can't speak on Braid having not played it but regarding Batman and Portal: Although I didn't bother trying to S Rank it I'd say Batman is easier than Portal. After solving the riddles and the finding statues I just had enough but I believe I could have done it. Still might go back and rent it again to clean it up. I have recently S Ranked the Orange Box and the Aperature Science Least Time trials are pure hell. I won't lie, I used a ton of You Tube (pausing every 2 seconds) and just sat down and grinded the medals out one morning. Extremely frustrating. I haven't been able to touch Portal since.

Modern Warfare is very easy.

" @Sudbuster: Just so you know: The only missable achievement in Batman: AA is one at the end of the game. This isn't really a spoiler, but just in case you really don't want to know...  

SPOILER WARNING: Click here to reveal hidden content.

The Joker throws you a 'party' at the end and you walk into a room full of guards that wont attack you. But if you attack all of them, you get an achievement.  

 
At least that's what I've read on achievement sites, I haven't actually gotten there myself quite yet. Maybe you've already gotten that one though, idk.

"