SHORT ANSWER:

Download and install this client and do not patch it.
UOAOS4.0.9.b torrent

Need a bit torrent client? Get Azureus

LONGER ANSWER:

The Angel Island server supports both old and new clients. The map files used on the Angel Island server are AOS 4.0.9b as these are really the ‘old school’ maps and don’t have the ugly additions that started appearing with Mondain’s Legacy (UOML.)

The perfect client:
Obviously the ‘perfect client’ is one that has map files that match that of the server exactly. That client is AOS 4.0.9b and is available below as a torrent download (no patching needed.)

UOAOS 4.0.9b torrent - (The perfect client):
http://torrents.thepiratebay.org/3503870/Ultima_Online_PRSV_uoaos4.0.9.b.zip.3503870.TPB.torrent

Other clients that work:
Any client that runs and makes you happy is fine; however PvPers and PKs will probably want to downgrade to a client version less than 6.0 because 6.0 introduces much ill-fitting deco at Wrong that will get in your way when you are PvPing.

UOML torrent:
http://torrents.thepiratebay.org/4052696/uoml_setup.exe.4052696.TPB.torrent

UOML direct downloads:
http://games.mirrors.tds.net/pub/ea-games/uo/
http://www.uogaddict.com/uodownload/
http://www.fileplanet.com/promotions/uomondainslegacy/

Super fast UOML Download:
ftp://largedownloads.ea.com/pub/uo/setup-1.46.0.3.exe

Untested downloads:

UOML
http://www.uo.com/uoml/dl_partners.html

Age of Shadows client (4.0.01):
http://www.mypulcinella.com/UOAOS_July_03.exe