Cyclic alkenes are subject to strain resulting from distortion of the sp-hybridized carbon centers.  Illustrative is C where the carbon centres are pyramidalized.  This distortion enhances the reactivity of this molecule.  Angle strain also is the basis of Bredt's rule which dictates that bridgehead carbon centers are not incorporated in alkenes because the resulting alkene would be subject to extreme angle strain.
Small trans-cycloalkenes have so much ring strain they cannot exist for extended periods of time. For instance, the smallest trans-cycloalkane that has been isolated is trans-cyclooctene. Trans-cycloheptene has been detected via spectrophotometry for minute time periods, and trans-cyclohexene is thought to be an intermediate in some reactions. No smaller trans-cycloalkenes are known. On the contrary, while small cis-cycloalkenes do have ring strain, they have much less ring strain than small trans-cycloalkenes.          
In general, the increased levels of unsaturation in alkenes leads to higher ring strain. Increasing unsaturation leads to greater ring strain in cyclopropene.  Therefore, cyclopropene is an alkene that has the most ring strain between the two mentioned. The differing hybridizations and geometries between cyclopropene and cyclopropane contribute to the increased ring strain. Cyclopropene also has an increased angle strain, which also contributes to the greater ring strain. However, this trend does not always work for every alkane and alkene.