It depends. If you want one on contract, and you want HSPA+ speeds (generally 3 mb down, if you're lucky) I'd suggest the Focus Flash. I think the Windows Phone OS pulls off smaller displays better than any other operating system, so the smallest display you can deal with means lighter weight, which is one of the most underrated aspects of a phone. From all the reviews I've read (and I read a LOT of phone reviews for some reason....) the Titan and the Focus S are both good phones as well. The problem I have with HTC phones is their heft and really bad battery life. I'm sure the Titan is great but I'm hesitant to trust HTC again after lugging their brick Inspire around for over a year.
If you want one off contract, I've read the Venue Pro is a nice combination of decent screen with physical keyboard and can be bought brand new off eBay for under $250. Personally, I've got the original Focus - its the only Windows Phone I'm aware of that allows you to put a MicroSD card in it for expansion. I've got enough music and podcasts that I need the extra space, and the original Focus has the best screen I've seen on a phone (other than the iPhone's retina display).
Plus, the advantage of the original Focus is that you can find them used in good condition cheap enough that you can take the plunge off contract for around $100 and if you hate it that much you can always use your contract to get a phone you really want. Resell the Focus and you'll probably get all your money back.