Any recommendations for a good creamy salad dressing?

Blue cheese: good cheese, mayo, lemon juice, salt, pepper and milk to make it the right consistency.

Ranch dressing is a classic. And easy. Equal parts mayonnaise and buttermilk. Finish with fresh herbs. I once met the guy who invented it, Steve Henson of Hidden Valley Ranch. He gave me a souvenir pen but wouldn't give up his recipe.

sometimes I add yogurt or creme fraiche to a simple vinaigrette.

this one is a little different and very tangy and good:
http://food52.com/recipes/3544-sweet-and-spicy-horseradish-dressing

p.s. In case it's helpful:
Greens with sturdy leaves - like romaine, iceberg, kale, etc. - hold up a little better to these creamy dressings compared to varieties like arugula, raddichio, etc., which get kind of drowned by a creamy dressing.

Not sure what your salad ingredients are, so some of the herbs might not be a good fit (or swap them out with something else?)

Basil Buttermilk Ranch Dressing
http://food52.com/recipes/22308-basil-buttermilk-ranch-dressing

Green Goddess Dressing (you could sub out some of the acid in this for more cream, sour cream or creme fraiche)
http://food52.com/recipes/23729-green-goddess-dressing

Thousand Island (just extract it from the recipe - it would work fine on salad)
http://food52.com/recipes/23915-grilled-leeks-red-onions-with-tomatoes-and-thousand-island-dipping-sauce

I sort of learned this from The Homesick Texan, Lisa Fain:

Take 1 cup of buttermilk and blend with 1/3-1/2 cup sour cream or Greek-style yogurt,~1-2 tb mayo (optional), and a bunch of fresh herbs like parsley, chives, thyme, & some jalapenos. Blend until smooth. Season with salt and pepper. It's a creamy tangy dressing and I love it with crispy lettuces like iceberg and romaine.

http://food52.com/blog/6790-buttermilk-ranch-in-a-jar

Look up recipes for green goddess. Also any blue cheese based creamy dressing.