It is probably best to jump into their conversation ask them why they think it is the best. Personally, I prefer -3's and -4's. That has everything to do with MIM, no lock, and proper tolerances. I like it that way. My opinion has nothing to do with the fact that S&W has been producing quality 629's for many years including -5's and -6's.
Mark
From what I can pick up in the online conversations, I think the guys touting the -5 as "the best" say so because they have one

I have one one as well, 6.5" full lug, and personally have no issues with s&w mim parts or the frame mounted firing pin. I've also owned a 29-3 in nickel and thought it was a fine revolver as well. I will admit that the -5 is more accurate than the 29-3 I owned, I just wouldn't say that because of that the -5 is "the best" nor would I say it just because I own one.

Speaking of which, a little rant here on the -4 and -5. I bought my -5 a few months ago after looking long and hard for a 6" full lug in my parts. Nothing came up, so when this one did, I jumped on it for $679 out the door. Well, guess what. Just yesterday a guy close to where I live put a -4 up for sale for
$575 with a soft case, two speed loaders and 50 rounds of ammo, some of it black talons

I really wanted to buy it as well, but I couldn't get the motion to pass the finance committee (wife). I guess Smiths are sometimes like playing the stockmarket, hard to know when to buy and when to wait!!
And really, all that said to say this: I like the upgrades in the -4 and I like them in the -5. I couldn't say there was a "best" between them, just personal likes and dislikes. In fact, I've never seen a "terrible" 629 in any generation from no-dash up.