I'm an original art fiend, so my listing will only apply to that aspect of the cons.
1. Emerald City Comicon-Great city, convention center is close to everything, ton of awesome artists, year after year. I've also met several people there that have become year round friends, so this con has a special place in my heart. I don't have a single complaint about this con...well except that it only lasts for two days.
2. Baltimore Comicon- All of the positives of Seattle, many different artists because it is on the East Coast; which is also the only problem, it's the other side of the country, so it's a much more complex trip (more time off, usually a more costly ticket, a bit more exhausting).
3. SDCC-The thrillamagilla of them all, but artists are getting the short end of the stick a bit more each year (cost is going up, space is being allocated, convention being less and less about comic art). With the incresing likelihood of this convention going to another city in the next 2-4 years (Vegas or L.A.), the chances of me picking another city (Toronto, Orlando or Charlotte) increase as well. Vegas and L.A. just don't appeal to me climate wise (I live in AZ, I don't want to travel to another hot friggin' city), and I've done San Diego over a dozen times.
The work has become more difficult to acquire commissioned art at SDCC, and most of the artists have to charge more every year, just to cover their expenditures. The cost saved by the close proximity of SD to AZ is pretty nonexistent at this point, so going back east, for artists that don't travel westward, gets more appealing every year.