I'm partial to Codename 47 while the series has advanced since that it still offers some great levels...Especially the last level mowing down all of those you know who...
I found the last level next to impossible!
Somewhere on the level (can't remember where) there's a minigun you can destroy all of them and not bat an eyelid with that :D --- Some idiots on GFaqs need to learn the difference between trolling, and a different opinion. If you're offended by this sig, you're one of those idiots.
I heard there were some licensing issues preventing them from putting Contracts on Steam. Don't have a source though.
I couldn't get through Codename 47. Got about halfway through the campaign, decided I didn't like it, and put it down. Silent Assassin had already been released at the time, so I moved on to that, loved it, and never looked back.
I'd say Silent Assassin is my favorite, with Blood Money and Absolution tied at second. --- "So my lunch today involved both a noodle packet and a sauce packet....so yeah, things are going pretty well for me."