Which one is the most common reaction in organic chemistry: SN1, SN2, E1 or E2?

You could write whole chapters in an organic text to answer this question. And of course what follows is merely a guide to further reading. We know that $S_(N)1$ reactions...$"substitution...

In a $S_n1$ reaction the reaction rate does only depend on the concentration of the substrate. Mening that you can add all the nucleophile you wan't but the reaction wont...

SN2 (substitution, nucleophilic, bimolecular) takes place in a single step without intermediates when a nucleophile reacts with the substrate(e.g. alkyl halide) from a direction opposite opposite to the leaving group....

.......whereas $S_N1$ processes are essentially bond-breaking reactions., and of course, are only dependent on the concentration of the electrophile. A bond making process at a tertiary carbon centre, for...

1-more stearic hindrance, more chance to give SN1 Rn. vice versa 2-more hyperconjugation capability of a substrate, more stable will be the carbocation ultimately more chance to give SN1 Rn....

Not only the solvent The type of nucleophilic substitution is decided by nucleophilicity and basicity of the nucleophyle, steric hindrance around the putative carbon, and type of solvent, protic solvents...

The clearest difference is in their names! SN1 reactions are unimolecular, stepwise mechanisms, where, $"R" = k["LG"^(-)]$ SN2 reactions are bimolecular, concerted mechanisms, where, $"R" = k["LG"^-]["Nu"]$ These reactions are...

Now of course $S_N1$ and $S_N2$ reactions are DISSOCIATIVE (i.e. bond-breaking), and ASSOCIATIVE (bond-making) respectively to a first approximation. Associative mechanisms are largely restricted to methyl, primary, and more rarely,...

The point is that YOU DON'T KNOW. And for this we apply $"a posteriori"$ reasoning, argument AFTER the fact, rather than $"a priori"$ reasoning, argument BEFORE the fact. Mix...