Exponential example for simplex used in SMT solvers
Here, you can find hard examples for DPLL. 
http://www.ioc.ee/~tarmo/tday-torve/itsykson-slides.pdf

For instance, it can be very slow for Tseitin formulas and for some unsatisfaisable formulas (it will try many solutions). 

I hope it will give you some hints :)