The quality of evidence increases and the number of studies decreases going from the bottom to the top of the pyramid. Starting at the bottom:
Animal research/Laboratory studies
Case Series/Case Reports
Case Control studies
Randomized Controlled Trial
Systematic Review: Articles that are Systematic reviews are desirable because they usually focus on the clinical topic with a specific answer and an extensive literature search is conducted to identify all studies with sound methodology. In other words, systematic reviews do the work for you by gathering many research studies together, analyzing them and presenting you with sound, evidence-based results.
Meta-analysis: A Meta-analysis will thoroughly examine a number of valid studies on a topic and combine the results using accepted statistical methodology as if they were from one large study.