The SSL and the Smart ones are very similar. Both are quite aggressive, but the Al Smart ones are a little cleaner, clearer to my ears, with the SSL ones being a little dirtier, with more crunch. Both have their uses, on the mix bus as well as drum busses or guitars.
The API 2500 is fatness for days, an amazing glue tool that sounds phenomenal on guitars and drums as well as the mix bus, too. It has quite a bit of color, yet can do most different styles.
The Crane Song STC-8 is nice, but very very clean to my ears. I prefer something with a little more color and personality on my mix bus, but many like to add color on their individual channels or groups instead of the whole 2-bus, so it's really up to you.
Obviously, it'd be best to own them all, but that won't happen, so I'd recommend you try them all out. If you can't, I'd personally go with the SSL or the Smart ones, just because they get used the most when I have access to them.