Voxengo Elephant
My first album was an instrumental metal album with electronic instruments and orchestra crammed in and I was at wits end trying to get it up to current volume levels and not destroy the sound. I tried a number of other popular compressors\limiters that cost much more and could not get the sound I wanted. I finally tried the Elephant and I couldn't believe how simple the interface was...and how well it just worked! I've since recommeneded it to others, one friend who was in a punk band came back to me and said, "That plugin worked miracles!". I literally use the Elephant at the end of my chain for every project now. It's a given.
I also bought Soniformer that same day and I've been very happy with it's performance. It's one of the only EQ\multiband compressors I trust to hand delicate high frequencies without distortion. It's extremely powerful, though the serious control provided makes for a somewhat confusing interface. Also I wish it provided for seperate left\right channel EQing\Compression. (of course that would clutter the interface even more haha!)