Welcome back to Demise! I havn't been playing Uo for a few weeks myself (due to life :/)
The guild you would want to join depends on what aspects of UO you want to enjoy. Here is my 2 cents (actually worth much less).
HZ: Mostly a raid guild. If you want to make lots of cash as fast as possible, HZ is the way to go. They are the biggest and the most organized. They generally win most spawns (not including despise). The guild leader MB isn't a bad guy but he's quite eccentric. He is an excellent guild leader, great at keeping trouble-makers quiet, and a good channel commander. One of the best if not the best guild leader on demise (efficiency!). If you don't mind being a zergling, listen in channel, playing only a spellweaver-necro, it's not a bad guild at all. Contrary to forum opinion the guys in guild are pretty cool too.
Not very active for non-felucca activitiews such as peerless etc.
If you do not join HZ and like to spawn, you will hate them until the end of your days (they will raid you almost every time you spawn in superior numbers).
A large portion of the members don't even play until MB's skypes "RAID" then the channel fills up fast(normally).
SEGA: The 2nd major guild on Demise. I don;t much about them They seem to have periods of inactivity (they might play other games I don't know). They were able to hold there own aginst HZ in a few fights. If you want to be part of "the resistance" and don't mind being the underdog, this would be the choice. They chain despise sometimes.
BOS: Since I left UO they have been actively recruiting (I don;t know if it was succsessful). They seem to be tight-knit, non-US players. When I was in HZ they spawned a very healthy amount. They would fight until the end, even aginst superior numbers, and didn't seem like greifers. I don't know too much about them but they seemed like a decent guild. they were CONSTANTLY trying to sneak T2A spawns. They have some decent 1V1 PvPers but I think they are almost all archers. They play on a non-US timezone I'm sure of that since raiding them is often outside of USA prime-time (4-11am est seems to be the active period).
ZS: mainly (all?) polish players. They have TONS of drama and duke it out publicly on the forums regularly. Alvin is sometimes a member and other times not. Be careful, Alvin(if he is ZS today) would kill his own mother for some mongbat-loot, HATES americans, and is a known scammer...Avoid him like the plague(I assume your american since your on eastern).
The guild leader posts on the forums sometimes and doesn't seem like drama though. If you speak polish or whatever they prefer, may not be a bad fit for you.
Good Luck!
(Disclaimer : I was in HZ for a few months, and raided all the other guilds multiple times so take that into perspective)