Well, as KF says, the Samsung 2500 can't be made multi-region, and it's now discontinued, so on past form unless you really must have a Samsung, I'd leave that one.
Whilst the Pioneer has the most options, it does not yet decode DTS-HD to PCM internally, we are still waiting for a firmware upgrade (I have a 51FD and a Sony S350) to enable it. Latest rumour is sometime in June, but we've been waiting a long time, almost 9 months, and at that rate it could be June 2010.
The Sony S350 and the Sony S550 have the same PQ, and personally I prefer the picture on the Pioneer. I think it gives a little more detail. However, at 1080p/24 whilst differences are there, they are not massive, and I'm 50/50 on whether the Pioneer is a good buy if you're not using analogue outs...
If 25% of your DVD collection is US based, and your TV will accept 1080p/24 over HDMI, then I would go with the Panasonic. The main reason is that for NTSC film based DVD's, it can get the original 24fps off of disc for you resulting in losing the judder associated with 60Hz playback. So it could breathe new life into your US DVD's.
Panasonic players have also been very reliable generally playing discs and well supported with firmware, so I think it's a safe bet for now, and can be made multi-region easily (as can the Sony and Pioneer) to cater for your US DVD's. None of them can be made multi-region for BD without hardware modification.
EDIT: Beyond this price range things really get more difficult and I'd say at the moment there is a bit of a gap. Some new Denon's the 2010 in particular are due mid-late summer and may provide something extra, but that won't help for your weekend. A european Oppo is due, again probably late summer, early autumn, but it's unclear if the European one will have any MR DVD capability. New Pioneers & Sony's are also due, but again not around now.
The next jump up is a big one, Denon 3800 etc, and if you're not using the analogue outs then that's where a large amount of the engineering in these players has gone, so unless you must have the best, their value drops a little for HDMI only use. I would stick with the cheaper players for now, and if you want to upgrade in 12-18 months time, then there should be much better machines and more choice around at mid-market price levels IMO.