I think Shula's at the Dolphin is the best. If you're limited to restaurants that accept the dining plan, though, the Yachtsman Steakhouse at the Yacht Club is a good bet.
For our money, Le Celier isn't even close to the best steak on property. We love YSH, but have also enjoyed the steak at Jiko, Artist Point, and even Flying Fish. Haven't tried Shula's yet, but the vast number of good reviews of that place would entice me to try there some time if it weren't for the connection to those Miami Dolphins....
We love the Yachtsman's Steakhouse @ the Yacht Club. It was so good that we ate there twice in one week last year(after a thunderstorm prevented us from walking to Epcot). The filet & ribeyes are awesome. Loved the mushrooms,too Excellent service. You will love it.
I have enjoyed the steak at Coral Reef. It is a one credit restaurant. Try getting into Le Cellier at Lunch. That is what we have done. Slightly smaller steak, but a really good way to spend one credit.
California Grill and Artist Point are two options that I don't think have been mentioned -the variety, service and food are excellent. Not to mention, both have excellent wine lists.