My take, having owned two of the five rifles listed and shot the remaining three:
Overall Quality:
Tikka/Bergara tie, then RPR/Christensen tie, then Savage
Aftermarket Upgrades:
Bergara/Christensen (both R700), then Tikka, then Savage, then RPR
Now, by brand:
Bergara:
The upgradability is always there because it's an R700 clone. Good quality and solid build. I had the pro model in 6 CM which has a Triggertech Diamond trigger and fluted, stainless bolt. I really enjoyed the rifle. Sold it when I got my 6 BRA. Bergara barrels are among the better (best?) factory barrels, but they are also proprietary.
Tikka:
Very smooth action. Upgradability is good, but not what's available for R700s/clones. They shoot well and are solid.
Christensen:
A friend bought one despite me counseling for something different. I helped him break in the barrel and get a scope mounted. It shot fine, but I wasn't overly impressed by the overall quality. My sense was that it would not hold up under hard use - no first-hand knowledge of them not holding up, however, just my general sense after playing with it.
RPR:
Another friend had one of these in 6.5 CM and I put a fair amount of rounds through it. It shot well until it didn't. Neither of us could figure out why it stopped shooting well. Shortly thereafter, he had a custom rifle done. "I wish I would have just done this first."
Savage:
I had a Savage in 300 WM. It shot just okay. When the 300 PRC came out, I rebarreled to that caliber with a Bartlein. It shot lights out - again, with a Bartlein on it. I turned it into my "frankengun" where I tried a bunch of different things on it. But the quality simply isn't there and it annoyed me too much. Now, it just sits in my safe. I have likened it to my Jeep. It works when it works, then it doesn't. Also, like Jeeps, some work great out of the box, some don't, most have these little quirks - mine has this annoying little buzz in the firewall when the engine is under load going up a hill at like 3k rpm - the Jeep, not the Savage