We went with the Perfect Harmony topper that CW sells. It has a base of 2" with memory foam and then another 2" of poly fill on top. We bought it based on the reviews that said it is cooler than an all-foam style.
We are very happy with the comfort. I also appreciate how it has a full skirt for keeping it on the mattress (and it makes it easier for me to put a fitted sheet on it if the topper isn't moving around).
The only downside we've had is it tends to compress where we sleep, so there are 2 long indentations with a bit of a mound in between. Not a big deal for all the additional comfort it provides.