It's really a case of taste and how you want your pre to sound. It's also worth noting that all of these channel strips include unison pre-amp technology. It's the most versatile across all sounds and anything you through through it will come out sounding great. If I were to say get one, get the SSL 4000 E Channel. I know this doesn't help a beginner who is looking and getting one and believe me you only need one. I personally find myself using the SSL's compressor most, the API's EQ and the Neve's gate.
The 3 other channel strips are really and truly all the same with a slightly different sound to them. This is a good option if you are on a budget and don't want to fork out the high price for one of the big boys. Not because it's bad, more because the others are so good. The Century Tube is the least of my favorites, maybe this is because it's the newest and hasn't been in my signal chains that much over the past few months. The EQ above this can be used to great effect to brighten the audio track without generating any harsh frequencies. Turning this to an 11 o'clock position almost gives your sound and instant 3D effect. The main features I find that make the Ampex ATR-102 such a brilliant and widely used plugins are the bias control. You will use both but believe me you'll find yourself using the Ampex more. Having both the Studer and Ampex is a luxury. I have all 3 of the tape emulation plugins and the one that gets used in every song is the Ampex. Put up against the Ampex ATR 102 though and I personally find it falls short. The Studer is good, it sounds great and it will do everything you want a tape emulation plugin to do. Honestly though don't bother go straight to the Ampex and Studer. The Oxide Tape Recorder has it's place in your setup but it is far from the best and should only be purchased as a luxury extra or if you want a tape emulation plugin on a budget.