Choice Scarf is a rather peculiar item. Contrary to what some people may have said in the posts above, I actually do believe that having a Choice Scarf Pokémon is pretty much a necessity for just about any competent team, regardless of whether it is offensive or defensive, as so many threats exist that even the most defensively-oriented of teams cannot possibly deal with even the vast majority of them using walls along, at least from my experience, so pretty much all teams will invariably need to depend on revenge-killing at some point or another. In fact, I personally believe that Choice Scarf is the one and only item which almost always needs to be found in a good team.
However, strangely enough, I do not think that this makes Choice Scarf the best item, as while I see having it in a team to be almost a necessity for its success, I find that having two or more Pokémon in the same team holding this item is actually detrimental to its success, as when they are locked into the wrong move, Pokémon holding this item tend to give ample opportunities for set-up sweepers and users of entry hazards to come in against them and begin boosting or laying down entry hazards, especially considering the fact that Choice Scarf Pokémon do not receive Attack or Special Attack boosts. The debilitating disadvantages of using two or more Choice Scarf Pokémon in the same team are especially apparent when one is facing a stall team, as against such teams, many Pokémon heavily favor either the 1.5x power boost of Choice Band and Choice Specs, or the ability to freely switch attacks, as at least one of the aforementioned is often necessary in order to break walls. In fact, along with possibly Choice Band and Choice Specs, I believe that Choice Scarf is the only item which shouldn't be found more than once in a good team.
As for the best item, I personally think that it is Leftovers, as both offensive and defensive Pokémon alike are able to benefit from it. In one extremely successful Übers team of mine, I even use Leftovers over Choice Scarf on Palkia of all Pokémon, as I think Choice Scarf is incredibly overrated on this Pokémon due to the fact that Ferrothorn can easily come into it and set entry hazards or use Leech Seed or status moves if it is locked into 3/4 of its moves (and the only move with which Choice Scarf Palkia can actually touch Ferrothorn is one which it would not want to be locked into in the rain, so the user of a Choice Scarf Palkia would be hesitant to switch Palkia into Kyogre and then Fire Blast the opponent's Ferrothorn switch-in, something I often do with my Leftovers Palkia, as such a move usually puts me at a great advantage in battle), as well as the fact that Arceus-Normal, by far the deadliest sweeper in the entire game, can easily set up on it when it is locked into any move barring a Water-type attack in the rain (having 100 base Speed, and therefore the ability to revenge-kill Dragon Dance Rayquaza is not a good enough excuse to put Choice Scarf on Palkia in my opinion, as Terrakion, a Choice Scarfer holder that can revenge-kill Swords Dance Rayquaza and much more importantly Arceus-Normal can also do that, and the same applies to Choice Scarf Genesect, which can also revenge-kill Swords Dance Rayquaza, Mewtwo, Latias and Latios, and the unbelievably useful Multiscale Lugia also exists to counter any Rayquaza anyway). Not only do I think that having the freedom to change attacks is extremely valuable for a Pokémon like Palkia whose greatest strength is its awesome offensive coverage, but having Leftovers recovery, in addition to Magic Bounce support in the form of Espeon to prevent it from being weakened by entry hazards with every switch-in, allows it to check and hold off the extremely dangerous Kyogre for a significantly long time throughout a match, while still being able to easily threaten Ferrothorn as previously-mentioned, unlike Latias and Arceus-Grass, the other two most common Kyogre counters, which easily give Ferrothorn free opportunities to set up entry hazards.
