As a matter of course most pedals usually come with cleats with a certain level of float. It usually comes down to a conscious decision from the user to go to a fixed pedal, if they so desire. That means that a significant majority of riders use floating cleats without thinking about it. In terms of set-up it is also usually easier as the floaters have significant margin for error in cleat alignment.
As above Speedplay have the most float, but the ability to actually use that much float is pretty limited. The ubiquitous Look pedals can be had with have 0, 4.5 or 9 degrees of float (depending on cleats). 9 deg is more than enough for me to whack the chainstay with my heel (in other words it would be impossible for me to use more than 9 deg of float - your mileage may vary).
If you have had no medical advice in this regard, don't automatically assume that float will cure your problem. There are a couple of reasons for this:
1) Your ankle and hip can both rotate in many directions, meaning that your knees will go where they want (within reason), irrespective of how much float your pedals have.
2) Usually your muscular balance around the hip has a far greater impact on the tracking of your knees (which should ideally follow a consistent plane of motion throughout the pedal stroke). Gluteus medius in particular.
People who have had knee issues, and have persevered, have found the solution to their knee issues in the form of many different combinations of cleat, shoe, pedal, insole and exercise regimen (to provide the required muscular balance). Most of us have no significant issue so we continue to use the cleats which our pedals came with and whatever shoes we like the look of. Others have found, through testing, that floating cleats are indeed good for their particular issue, while others, still, have found that fixed cleats actually work better for their knees.
The key is perseverance. In other words, whichever system you opt for, and like any other facet of bike fitting, you must be prepared for a certain degree of experimentation.