I performed and IPA analysis on differentially expressed genes. It will give me log10p and ratio on y-axis vs list of pathways on X-axis. My question is is there any rule by which we can say one pathway is most enriched or not. Like in terms of the p-value of number of genes in the pathway.
There is no one "right" rule, but a rule that is typically used is adjusted p-value < 0.05.
The p-value should be adjusted for multiple comparisons (usually via false discovery rate [FDR] control). Don't base your 0.05 (or whatever) cutoff on unadjusted p-values please.
Edit: If you mean which pathway is MOST enriched, usually softwares provide some sort of enrichment score and you can choose the pathway with the highest enrichment score. If you're only giving the p-values, then a heuristic would just be choosing the pathway with the smallest p-value.