IMO, I like Le Cellier's mushroom filet mignon the best. I'm not a steak expert so I rarely pay attention to "quality" (marbling, corn-fed vs. grass-fed, broiled vs. oak-grilled, dry vs. wet aged, etc). For me, if it's cooked to my liking, seasoned well, and mostly lean, I'm happy with it. What I like about Le Cellier's filet is the truffle butter served with it, and the creamy risotto.
In second for me would be Yachtsman's filet. It was also cooked very well, but I think red wine sauce and mashed potatoes are less unique (I've made that at home). The Yachtsman porterhouse was way too fatty for me (I hate chewing on steak fat), but that was my mistake in ordering a cut like that.
Never been to Shula's. If I ever really want to, I can try it in NYC, so it's not a must-do for me at WDW.