I use Mizani Butter Blends Lye in Medium/Regular! My hair is SUPER fine. Like, I look at my strands and wonder how they don't just melt off at the mere sight of a relaxer tub.
Mizani BB leaves my hair feeling thick (I have that weighed down feeling after relaxing), silky and shiny as heck. It's also really strong. I never seem to have breakage the week after I relax and daily manipulation breakage is pretty normal for me otherwise.