Based on what we see in the movies, the physically weakest Predator has to be FALCONER. That Predator didn't show us anything that implied any type of impressive physical strength, exceptional endurance etc. and on top of that Falconer went down after being cut ONCE by a human with a human-made sword.
Sure, Wolf had it really easy as he faced the dumbest, squishiest, most sluggish and passive blob-like Xenomorphs ever shown on screen, and the humans were dumbass meatbags as well, posing no threat at all. Even so, Wolf showed some feats of strength, lifting xenos by the neck, fighting a Predalien etc. So although Wolf is probably the dumbest, most inefficient, clumsiest and most incompetent Predator, facing the the lamest opponents imaginable, I'd still have to admit that Wolf doesn't qualify as one of the weakest ones we've seen so far.
I also find it unfair to list Chopper and Tracker as they didn't really get to do anything before they were killed. When it comes to Fugitive it's tempting to label that one as weak due to the way Upgrade manhandled the poor thing, but before that we saw Fugitive ragdoll a bunch of humans in that lab. I'm not going to count Original (from Predators) as that poor sod was a wretched and tattered prisoner of war, tortured and whatnot, just waiting to die (yet still put up a fight).
I'm deliberately not going to count Upgrade as Upgrade was a cheating steroid nightmare packed with bionic/biotech enhancements.
From a raw display of power kind-of-view I'd say that CELTIC qualifies as the Predator with the most brute strength. That fight with Grid was epic and Celtic made the whole place rumble, shake and shatter when he literally WWF brawled with a super aggressive and exceptionally cunning Xenomorph. However, Celtic still went down kind of easy compared to Jungle Hunter, City Hunter, Feral and Berserker, as all of them got severely mutilated, shot, pummeled before they finally checked out. Even Celtic's heartthrob hunting buddy Scar took more of a beating before finally dying.
So figuring out who's the strongest is a hard one. Yes, Celtic is probably the strongest when it comes to explosive raw strength, but would Celtic be able to keep up if the fight gets drawn out? City Hunter was a complete psycho and basically vaded through death, throwing people around, got an arm cut off and got cut open with a smart-disk. Jungle Hunter was more stealthy and careful, but that bastard got shot plenty of times with heavy caliber weapons, ragdolled Arnie and didn't die right away after getting a giant log dropped on one's head. Feral wrestled, killed and power-lifted a freaking grizzly bear with ease, and just like Jungle and City Hunter, and Berserker, Feral kept going even after being shot and mutilated. Berserker ranks lower because there was no real display of raw power. Sure, he took down Original, but Original was in a severely weakened state.
With all of that in mind I'd say there's a tie between Jungle Hunter and Celtic when it comes to who's the strongest Predator. Weakest is Falconer by far.