The top Cataclysm achievements for guilds and solo will vary between players and guilds but there are certainly some that stand out above the rest. The new guild achievements require guilds to work as a group and the rewards are certainly worthwhile. Players can still benefit from solo achievements when they are not busy helping the guild progress.
Insane in the Membrane is a solo feat of strength that is still very sought after. The achievement has actually been made slightly easier with the removal of reputation for the Shen'dralar. This reputation has been removed from the game and consequently the achievement whilst the other criteria remain the same.
Glory of the Cataclysm Hero and Glory of the Cataclysm Raider both reward a mount, making them some of the most sought after achievements in the game. These achievements can be earned without a guild, however the guild versions give different additional rewards. Both guild and solo achievements can be earned at the same time.
Guild Cataclysm Dungeon Hero rewards the Standard of Unity which allows you to place a banner that increases experience and honor gains for all guild members within range of the item. The Guild Glory of the Cataclysm Raider offers a dark Phoenix mount. The solo versions offer a Volcanic Stone Drake mount for the Heroic version and Drake of the East Wind mount for the Raider version.
We Are Legendary is sought after simply for bragging rights. It is a guild achievement that requires the guild to obtain, whilst in a guild group, the legendary weapons available within the game. This achievement does not count weapons previously obtained but requires the guild to obtain them as a group.
Critter Kill Squad is a fun guild achievement designed for the critter killers among you. It requires a group effort of slaying 50,000 cute fluffy critters and the reward at the end of it is the armadillo pup vanity pet. Pet collectors will want to join in this one, even if it goes against their love of the cute little critters.
There are many more achievements within the game, many around professions, PvP and reputation but these are the ones that stand out as the top Cataclysm achievements for guilds and solo. Whether you prefer to work alone or with your guild, there is always plenty to work towards and don't forget that some achievements can double up and earn you the solo version as well as the guild achievement at the same time.