There are many reasons as to why one particular Pokemon can't be chosen as the best.
1) Different roles
utility (Blissey [wish passer] & Forretress [status inducer/hazard setter]), sweeper (Scizor, Aegislash), sweep blockers (Sabeleye, Ditto, Wobbuffet)hyperoffensive/wallbreaker (Lucario, Machamp), wall (Snorlaw), weather inducer (Politoed)
Vaporeon (Wall + Wish passer), Hippowdon (Wall + weather inducer) Cloyster (Shell Smash sweeper OR hazard setter)
2) Theoretical values
Cumulative effectivity statistics - Taking the effectivity of moves into account, this is a number calculated measuring the efficiency of the dual type combination as a defender. In other terms, the lower the number the more resistant to other offensive move types therefore superior to other competing types in terms of defense. As we can see Steel/Fairy is the most efficient defensive typage. With Steel-types generally dominating the resistancy charts, on the other side we see Grass/Ice/Rock-types failing to be good defensive types. If you've ever played competitively, you probably have been conditioned to switch out as you realize you have a Grass/Ice/or Rock-type out on the field.
Single weakness Pokemon
Normal types only have 1 weakness: Fighting
Since Fighting moves are VERY commonly physical, most Normal walls are specially defensive. A great example is Snorlax, Munchlax, Audino, and Lickilicky. All of which have recovery moves that further secures their job as a specially defensive wall.
Several of these single weakness bearing Pokemon are a key to effective teambuilding through implementing perfect synergy thereby reducing redundancy. Many times serving as double endendre.
BST (base stats total) & BSA (base stats average) - With the exception of the cocoon Pokemon and Pikachu, almost all 1st evolutions fall under 57 BSA with three and two stage evolution chains progressing towards the secondary evolution. Again, with the exception of the cocoon Pokemon, three stage evolutions progress onto the second stage at around 80 BSA. 87-90 BSA is where the starters lie. Anything above 90 BSA is a very proficient third stage evolution, legenedary, pseudo-legendary, or mega.
3) Tiers have a range of different Pokemon
and as tiers go upwards, the BST & BSA tend to rise as well. Thus, a single Pokemon can not be effective through all tiers.
But this isn't even the only answer because,
Although base stat totals and averages tell a lot about Pokemon, they can't be the single determining factor of a Pokemon's strength.
4) Special abilities, items combinations, and extremeties
ex: Shedninja (Wonder Guard ability), Ninjask (very high Speed BS), Skarmory (very high Defense BS), Krookodile (Moxie sweeper)
sources: "Pokemon Dual-type Charts." Pokemon Database. Pokemon Database, n.d. Web. 22 Mar. 2014.
"List of Pokemon by base stats." Bulbapedia, the Community-driven Pokémon Encyclopedia. N.p., n.d. Web. 19 Mar. 2014.
