This is a question that’s asked quite a bit for players returning to Ultima Online. For more on shard specifics, see the UO Guide page on Shards.
North American
Atlantic is the most populated. With it comes a bit of a housing crunch, and do you really want to have a smaller house? With that in mind, I’d recommend the following:
– Lake Superior
– Great Lakes
– Pacific
– Followed by Chesapeake, Napa Valley, Catskills, Sonoma (no particular order)
Lake Superior, Pacific, and Great Lakes could be interchangeable – if you have established characters on any of those, I would stick with them. They are generally the three most active of the North American shards after Atlantic, which is not surprising, as they along with Atlantic were among the first shards opened.
There are some really strong communities on Baja and Great Lakes, and if you are into roleplaying, Great Lakes has a large roleplaying presence on the web.
There is always Origin which gets publishes first.
European Shards
Europa is generally considered the most active/populated from what I’ve read.
Japanese Shards
All of the Japanese shards can be quite busy at times, and even if their shard populations are not too high, they still have very active communities.
What about Guilds?
Looking for your old guild or looking for a guild centered around certain interests? Check out UO Guilds by Rupert Avery.