Yeah... I know... I'm too late! But I got very busy this week and totally forgot that this week is one of my favorite legendary pokemon. I use one in my uber team. While I do agree, that mewtwo fell of grace since it's introduction back then, It is still one of the best special sweepers in the game. It's typing let's it down a little for lack of resistances and it's defenses are somewhat low in the uber tier (Same problem with rayquaza). Mewtwo compensate it's weak points with a movepool as vast as space and a speed stat so horrifying, that it can outspeed a very large amount of the uber metagame and hit them hard with that 154 special attack stat, again one of the highest in the game! With the introduction of new pokemon, threats have emerged, notably extreme killer arceus as it can take an unboosted aura sphere from mewtwo while doing serious damage to it with extremespeed. But even that is not sure of a counter as mewtwo can take an extremespeed and since most mewtwo's run life orb or expert belt, that aura sphere will be doing a lot of damage.
My personal set:
Mewtwo@life orb/expert belt
Evs: 252Sp ATT/ 252 Speed/ 4 Sp DEF or HP
Nature: + Speed / - ATT or + Sp ATT / - ATT
Movepool:
-Psystrike
-Ice beam
-Thunder / Thunder bolt
-Aura sphere
An all out special attacker set. Very useful, very effective and scary... I run expert belt over life orb because I hate the 10% HP loss and I cannot stand choice items! Psystike is for STAB and does more damage than aura sphere to chansey and blissey. Ice beam is for all those dragons that fill the uber tier especially rayquaza. Thunder is a good options in rain teams as it has riddiculous power and hits kyogre for a lot of damage. Thunderbolt is if you are not using mewtwo in rain. Aura sphere rounds out the coverage by hitting darkrai, and dialga hard. Dialga walls all your other moves!
EVS are obvious while nature is tricky! I used to run +Sp ATT / - ATT nature until I faced a darkrai. Stayed in thinking that mewtwo naturally outspeeds darkrai and aura sphere is more than enough for bringing it down. Until poof! Drak pulse and mewtwo got creamed! From that typical battle, I changed mewtwo's nature to boost speed instead just in case I faced another similar darkrai!
Mewtwo appreciates a little team support like double screens and why not a little special attack boost to be all great! I usually baton pass shell smash boosts to rayquaza with a focus sash that often sweeps entire teams with it if done successfully, but once I baton passed them to mewtwo. It managed to kill one opponent before arceus put a stop to it! It was a disaster! I lost that battle because rayquaza is very bad without the boosts!
All in all, mewtwo is fearsome, it rightfully deserves the title of legendary and most of the times it will do serious damage to opponent's team before falling down!