I'd get the one that draws you the most.
Here's where I stand, personally. I bought a ZT0350 on forum recommendations, and it's a nice knife, but it never gets pocket time, and will likely be sold off here soon. The Ritter Griptilian is the only knife that ever manages to kick my HTM Gun Hammer out of my pocket (and that only rarely, and only after I made custom scales for it). That's all personal preference though. There are plenty of people that really like their 350, and the Para 2 is an incredibly popular knife. I'd probably trust the 0350 the most for serious hard use, but I think both of the other two are probably better in terms of general EDC performance. ZT knives are quite bulky, and what they gain in "built-like-a-tank-ness," they lose in performance. That's not a whole lot, mind you.
A few things to keep in mind about each knife:
Ritter grip--great finish, won't show scratches as easily, has the AXIS lock, which is easy to open, and very strong. Excellent performer in terms of sheer cutting and slicing ability. Biggest con is the handle. Get one with custom scales (cuscadi and wilkins are the most popular options), and you'll have a seriously great knife. I kinda doubt you'll really love it until you get new handles for it though, just FYI.
Para 2--not much to say about this. I never got one, because I don't like how long it is. There's a LOT of handle for the amount of blade you get. I prefer a knife that maximizes handle/blade ratio. This one doesn't. Probably the best cutter of the 3 you listed, however. Maybe not hard use due to the blade profile, but the compression lock is very good, and it would make a solid EDC for MOST people. If you're into abuse, perhaps not.
ZT 350--I'd put the most trust in this one for someone who would abuse their knives. It's a hefty piece, by far the heaviest of the 3, and you won't be likely to forget you've got it in your pocket. Solid knife, but the DLC and the blade design seem to somewhat inhibit it's cutting potential, from my tests. I did a 5 knife comparison review, if you're curious why I say that. I can't really pinpoint what I don't like about it. I just don't ever carry mine. I think that might be because I just don't like the way it looks as much as I do the knives I DO carry, and if I lost my Ritter Grip or my Manix 2 or any of my Gun Hammer's, it would definitely find it's way into my pocket.
You've got a good lineup there though, and if you like the looks of those knives, you'll certainly be happy with any of the purchases. Pick the one that calls to you the most. That's always the best advice I think anyone can give. No matter how good the reviews, if you don't REALLY want a knife, you won't carry it, and you'll eventually regret buying it.