celebi is good for a number of reasons. it is good in adv in part for the same reasons it was good in gsc (but definitely not to the same degree because it's not as bulky and the meta is more offensive overall. plus it has better counters now). because of its stats, its ability, natural cure, and its access to recover and excellent support moves like leech seed, heal bell, perish song, and reflect, it is a great defensive pokemon and team player. with leech seed it can deter major threats like suicune and snorlax from attempting to set up to sweep early in the game, as well as heal itself and its team mates. perish song makes it a good counter for the last-pokemon set up sweeper strategy, and a decent phazer in the meantime. the benefits of heal bell and reflect are obvious. its typing is a bit of a mixed bag defensively, providing it with some good resistances, such as to ground, water, and electric moves, but also some key weakness to fire, ice, flying, bug, and dark attacks which aren't too difficult to take advantage of. the weaknesses serve to balance it out to some degree, but it is still an excellent counter to pokemon like zapdos and swampert. the real trouble with celebi, though, is that it can impact the game to a similar or greater degree with an offensive set. calm mind makes it an even better special wall when included on a defensive set, but it can also tear through many standard tss teams, which utilize pokemon like gengar, skarmory, swampert, forretress, claydol, etc., with its base 100 speed and special attack, and a combination of giga drain/hp fire/psychic. even without EVs invested in its hp and defenses, it's bulk is substantial enough to let it set up rather easily and stick around for quite a while. additionally, celebi has access to what is arguably the best move in the game in baton pass, which makes it excellent on trapping teams, as it serves as a perfect lure to pokemon such as tyranitar and skarmory for magneton and dugtrio to take advantage of. its capabilities do not end there, however, as celebi is also capable of passing calm minds and even swords dances to pokemon that will have an even easier time sweeping like zapdos or metagross respectively. its incredible longevity and ability to completely tear through or shut down an opposing teams in a number of ways makes it difficult to counter and a major threat to the metagame.
too long?