I always thought Mew was a bit mediocre in adv, it looked a lot like Celebi: same stats and largely the same moves. I definitely think it's a pretty bad pokemon for being Uber tier, but it would be insanely strong in OU. Hard to place I guess.
EDIT: Taking a closer look at its (adv) movepool, useful ('useful' is subjective) moves that Mew learns that Celebi doesn't:
Counter
Dynamic Punch (or maybe not)
Earthquake
Endure
Explosion
Fire Blast
Flamethrower
Focus Punch
Ice Beam
Overheat
Roar
Rock Slide
Seismic Toss
Snatch
Surf
Sludge Bomb
Taunt
Thunder Wave
Thunderbolt
Celebi:
Baton Pass
Heal Bell
Leech Seed
Perish Song
Additionally, Natural Cure is better than Pressure in the majority of situations. Then again, Mew has better typing. I feel like Mew is generally overshadowed by Mewtwo.
EDIT: Taking a closer look at its (adv) movepool, useful ('useful' is subjective) moves that Mew learns that Celebi doesn't:
Counter
Dynamic Punch (or maybe not)
Earthquake
Endure
Explosion
Fire Blast
Flamethrower
Focus Punch
Ice Beam
Overheat
Roar
Rock Slide
Seismic Toss
Snatch
Surf
Sludge Bomb
Taunt
Thunder Wave
Thunderbolt
Celebi:
Baton Pass
Heal Bell
Leech Seed
Perish Song
Additionally, Natural Cure is better than Pressure in the majority of situations. Then again, Mew has better typing. I feel like Mew is generally overshadowed by Mewtwo.