Now this is a topic that is very near to my heart, since it was an omnibus that bought me back to comics after an almost fifteen year absence.  I'd picked up a handful of trades during that time (the stuff that broke through and got mainstream coverage - e.g., Kingdom Come, League of Extraordinary Gentlemen), but I really hadn't given much thought to comics during that time.  Then one day about a year and a half ago, I was in Borders and came across a copy of Uncanny X-Men Omnibus Vol. 1.  It was an impulse buy, but I can point to the things that convinced me to plunk down $100 on one book when I hadn't spent that much on comics combined in fifteen years - a large chunk of classic stories, incredible production values (great restoration and, most importantly, SEWN BINDING) and it even included the letter pages.  Since the Dark Phoenix Saga was published before I was born, it was great to read the letter columns in the issues leading up to those books and see what the tenor of the fan discussion regarding Jean was like when people didn't know how DPS would play out.  I was hooked once again and really appreciate all the effort that went into crafting such a quality book.

So my number one most desired omnibus is Uncanny X-Men Vol. 2 (and on through Uncanny 280 - a guy can dream...).

But given when I came back to comics, I missed out on a number of now OOP omniboo.  I would love to see Frank Miller's Daredevil, Grant Morrison's New X-Men and Alias go back into print.  I would gladly pay the premiums these books are going for in the secondary market if they didn't have glued binding.  But I need sewn binding, so I'm dying for these books to come back into print.

Of the not-yet-omnibused, the ones I'd most likely to see are:

Walt Simonson's Thor
New Mutants (through the conclusion of the Sienkiewicz run)
Garth Ennis's Punisher MAX
Fall of the Mutants