After going through Sense (stock), LauncherPro, GO Launcher, Apex, and finally Nova, I have to give it to Nova for being both smooth and feature-rich. I was wary of switching from Apex because of all the customization it offers, but Nova has everything I loved from Apex and some other nice features, and overall seems to be the smoothest launcher of all the ones I've tried.
Same here. I tried TouchWiz, Motoblur, LauncherPro, ADW, Go Launcher, Zeam, Nemus, MXHome, MIUI, Apex, and Nova.
edit: forgot I also tried Launcher 7 and a few others for short periods. I tried probably 15 in all.
On 4.0+, I think the free version of Apex is better than free Nova because of the features. However, if you're willing to spend $4 to get all the Nova Prime features, it is definitely smoother with no FCs or other problems. I use Nova Prime pretty much exclusively now. It was great on ICS, and it has been great on JB.
For 4.0-, I think it depends on the phone. While Go Launcher offers the most customization, themes, etc. for free, it consumed more memory and battery than I liked on my 512 MB RAM Droid Charge. On phones with 512 MB, ADW works fine, but Zeam and Nemus are definitely the cleanest, fastest, and use the least amount of memory and battery. I used Zeam the last couple of months I had my Charge, and my dad has been using Nemus ever since I gave it to him.