Just putting my opinion (excluding legendaries)
Steel - Metagross. Awesome attack and defence as well as being a great pseudo-legendary
Dark - Umbreon...great defence and spec defense as well as looking awesome
Water - Milotic pwns :P
Fire - Arcanine. High attack and spec attack (would put charizard but it gets owned by stealth rock)
Grass - Roserade (Spec attack and spec defence is pretty awesome)
Dragon - Garchomp...obviously. Did you know it has an equal base stat total with Celebi, Cressilia, Darkrai, Deoxys, Heatran, Jirachi, Latias, Latios, Manaphy, Mew and Shaymin! It is in the UBER tier of the metagame, and is the best pseudo-legendary by far. Honourable mentions to Dragonite and Salamence.
Electric - Magnezone. 12 resistants and 1 unaffected + good spec attack and defence
Fighting - This is probably a contreversial decision but I think I am going to go with Hariyama over Machamp because of its high HP
Bug - Scizor. Dual type with steel makes it brilliant. Wins by a mile.
Flying - Skarmory has pretty damn good defence and because it is dual steel, it is not weak to ice nor rock
Ghost - Dusknoir. Low HP but more than covers for that with high attack, defence, and spec defence
Ice - Froslass. Average stats but high speed, dual type ghost and good movepool make up for this
Ground - Hippowdon because of its high HP, attack and defence
Normal - Blissey because its HP is !@#$%^& amazingly high as well as spec defence. Attack and defence are >.< so its best move is probably (when at level 100) seismic toss
Poison - Drapion is a slightly strange choice but I like it. Its only weakness is ground and it has reasonable defence so it can deal with that. You may have to rely on STAB though because of its average (but not bad) attack.
Psychic - Alakazam for me purely because I have used it since Pokemon Red (wel...kadabra because I didn't have a link cable but Wi-Fi is god in disguise). It has awesome speed and spec attack. Closely matched by Gardevoir though.
Rock - Tyrantar for me. Awesome pseudo-legendary and its dual type drak helps with moves like crunch
Hope this helps. :D