I used all fountain pen versions of the makers the OP mentioned, as well as the Lamy 2000 and a Rotring. If I had to buy replacements for them I'd get only the Waterman and the Lamy. Here's why:
The older Parkers are definitely better. I have a couple of the original 51's, and they are excellent. The newer models are very finicky about ink, and dry out unless used often. They are a real pain to clean when that happens. If you rotate your EDC among several pens, you can plan on cleaning the Parker more often.
The nibs on the Cross pens are very temperature-sensitive. I've found that makes them prone to seeping onto your fingers, or oozing ink into the cap (loads of fun in meetings).
Rotring was a tank, but it never really "wrote in."
The Watermans tend to take a while to "write in," but then they're great. I keep coming back to my Carrene.
The Lamy 2000 is a great pen, but you'll need a ink bottle (piston fill), and the pen is very lightweight for its size.
Sorry for the long post, but I'll add a few other thoughts. The obvious ones are thickness and weight. There's balance too, but that will change if you write with the cap "posted." Lastly, are you a heavy hand when you write? If so, whatever brand you get, consider whether the nib is "hooded" or free-standing. Generally, hood nibs are stiffer and more durable for a given width.
