Phoenix : phoenix_lj
Feature-laden, moderately active development. Has some bugs, but not of the crashing variety. Implemented in RealBasic, and it has some severe performance problems. Posting and editing longer entries (like, say, fiction posts) is better accomplished with the web interface than with this app. Which is sad. The RealBasic implementation also means that the text editing is not quite Mac-ish; I write all longer entries in BBEdit and paste them into the post window.
xJournal : xjournal
Cocoa. Active development. A bit crash-y. Has a nice poll editor. (Though once you learn the poll syntax, it's faster to write them by hand. As with everything. If you're me, anyway.) The user interface is better-designed than Phoenix in some ways (history browser) and not quite as good in others (post window is less compact). Probably the one to choose right now, if you can put up with the occasional crash.
iJournal : ijournal
Cocoa. No updates for about five months. Has a poll editor. Straightforward, decent posting interface. No history viewing or editing, but you can edit the last post. Open source. For basic posting, works great. Has probably the best friend editing interface of the three.
Ecto and MarsEdit both support LiveJournal through its Blogger API, but no serious LJ poster would use that API. No userpics, tags, moods, and so on. You want an LJ-specific client.
I just checked out the source for iJournal. I've been looking for a good programming project to get me deeply into Cocoa & Objective C. I did a little project last year, but it wasn't the kind of thing that I had a big incentive to continue. This might be it. (Don't worry; not this month. Maybe not at all unless I get bored.)