In terms of overall good artists, London Tattoo convention is the best show I've been to so far. Venue is big and organized, people are all very nice, and top notch work from all over the world.
Within the states I really enjoy Ink-N-Iron in Long Beach California as well as Hell City in Phoenix Arizona / Columbus Ohio. Both have some really good artists and brings something worth checking out to the table.
Ink-N-Iron has a great venue (on the Queen Mary Ship) along with good vendors, live music all day, and some great custom cars all in one event.
Hell City is just organized very well and always struck me as a consistently good show. The Arizona one is hosted nearby/on the premises of the Biltmore Hotel which is amazing. I definitely didn't enjoy the temperature of Arizona but the convention more than made up for it.