Assassins Creed is an amazing game and it does at times seem repetitive but is very unique and has a good story line. Maybe just a rental, and if you like the way it plays, and the supposable repetitive play doesn't bother you then buy it.
Call of Duty 4 is very famous for its online play, but the Campaign is absolutely amazing, even after the Campaign there are things you can look for throughout the levels to gain very fun cheats that no other game would offer (Ragtime Effect) and all the achievements you can acquire are in the Campaign, so if you beat the Campaign, you can always still stive for the harder to get achievements. Although not having online play will be a harsh downfall, the Campaign would be worth every penny.
Halo 3 is also another amazing game, although if you haven't played the first or second you may be a little lost with the story. And if you don't have the ability to hook up Live with CoD4 then I'm assuming you don't for Halo 3, which is another huge part of the game. Halo 3 also has about half of its achievements in its Campaign and the Halo universe is one like any other game, and is very easy to catch on to and become quite good at, but it will still have a challenge with it's Legendary missions, especially if you're going in solo.
I'd say, with your conditions, Call of Duty 4 is your best bet. Although it's not beating Halo 3 by far. Assassins Creed I never really got into, but the ending was amazing. Depends if you like stealth games, realistic first person shooters or fictional first person shooters. Get lost in them, they're all great.
