I've used all 4. I'll just give you an overview of what I think are the most important aspects. I'm sure I'll leave out many points. I do suggest you download and try the software.
I would rank it like this:
Resolve:
Pros:The most powerful of the 4 (in the market really!) for color grading/correction. The tracking/keying is probably the best in the business, working fast and accurately once you outline the target. The secondaries are unlimited (I think the free Lite version is limited but not sure) and because it's a nodal UI, you can run them parallel or serially, which allows multiple looks within the same edit. Great noise reduction with GPU realtime. I/O with all popular known formats native. Works with multiple GPU and CUDA acceleration (again I think the Lite version doesn't do that) for real time.
Cons: Hardest to learn and complicated if you are not experienced with node based software. Can be slow without GPU acceleration. System has to top of the line! Standalone app so there's the whole workflow thing to consider (at the end vs import/export to NLE)
I totally love this software! Amazing control and flexibility and allows you to progressive upgrade as needed. Comes with BMCC! On par with Scratch or Smoke as a software... but those software work much better in networks etc.
Speedgrade:
Pros: I really like the layer based interface, which is very much like AEFX or Photoshop. Supports the big camera formats like .r3d etc. Good selection of presets to get you started. Easy to add Secondaries to isolate problem areas and masks are easy to apply. Like Resolve, easy to save a preset and apply to other clips.
Cons: Dial/wheel controls were a little hard scrub without a control board, stupid conversion to DPX from Premiere Pro if you use send to Speed Grade (vs exporting EDL then import files). Tracker isn't very sophisticated and pretty slow to analyze the scene. Still version 1 so it doesn't fully integrate into CS6 yet. Does not support CUDA acceleration. Keyer isn't that great.
I wouldn't buy SG on its own but if I'm switching to Adobe, I'd buy the production suite and use it as a primary grader.
Colorista II:
Pros: Works within PP. Allows 3 way primary and secondary layer grading and the masking tools are pretty good. For underwater purposes, the Master HSL wheel tool is good, allowing a relative easy lowering of cyan coloration. Very simple to use as you can use the wheel or numerical values. Nice skin overlay for showing skin color. Open GL acceleration. Mask tracker is good. Nice keyer tool. SInce it's a plug-in, it's great as it stays within the NLE to finish up edit.
Cons: Only allows 1 layer or primary and secondary. Curve adjustment only on the primary (master) layer.
It's a nice program that does great job of color grading. However, with SG coming with CS6 Production, it's not as sexy now as an additional cost, unless you do a lot of keying. You may wish to buy the Magic Bullet Suite, which includes Colorista and Looks. MB Looks has an awesome tool to remove lens distortion. It's slow but to remove barrel and pincushion distortion, it's very nice. Don't think you can use it without PP.
Color Assist: Admittedly I only used it maybe 5 times.
Pros: Works nicely with DSLR files shot with Cinestyle profile. UI is easy to use and has all the scopes you need to properly grade/correct. 25 preset looks and customizable presets for looks.
Cons: Only primary grading/correction, otherwise have to use the key tool. Standalone app, so it's difficult to implement as a workflow like Resolve. Best used with flat profile like Cinestyle.
I don't think this is a great color grader/corrector as a product. I think it's more for a DP to preset looks for the colorist to follow.
Hope this helps.