The "best" acid is determined by the reagents you are using and what product you are trying to achieve. Sometimes, a protic acid of any kind works just fine. Other times, a Lewis acid works better if there are other groups that might react. MgX2 is often used for certain systems. I highly recommend this website for suggestions for all kinds of organic reactions. This following link is specific for aldol condensation showing both base and acid catalyzed. http://www.organic-chemistry.org/namedreactions/aldol-addition.shtm