Yes Darren, many people still play UO, and UO will be celebrating its 15th Anniversary this September.
If you have played UO in the past, you might be able to recover the passwords to your account(s) and be able to use the accounts and characters that you have used in the past. It could save you a lot of time and it might be fun to be using characters you used many years ago. Your characters may still have supplies, gold, etc. either on the characters themselves, or in their bank boxes.
As for which is the most populated - Atlantic, but keep in mind that if you want a large (18x18) house or a tower or other large structure, you will probably be much better off with lesser populated shards where you can easily place large houses/towers.
If I were you, I would see about reactivating my old accounts and then use the characters on those accounts to decide which shards you want to play. I would also look at UO Guide, UO Stratics, and UO Forums and read up on the individual shards and shard forums and pick your shard based on your interests and not necessarily which is the most populated.