Since I bought the Bear Montana, I have been doing a lot of shooting. I started out getting reasonably good groups, not necessarily in intended location, usually about 8" to the right at 20 yards, (repeatable but not accurate). Recently the repeatability went to hell my arrows would be all over the place. I observed that the rear of shafts had started pitching up when leaving the bow.  At this point, I noticed that the cheap rest that Bear used on this bow has torn and lifted up slightly.  I can only assume that this is the cause.  Anyway, I'm going to replace the arrow rest and side plate what do you all recommend for replacement. Calf hair, Bear hair rest/side plate, or just a leather rest and spacer.
Thanks in advance.

Any of the above should work well once your tuned back in, bear hair rest/side plate is a good choice,although I got a bear patriot I just use some moleskin on, personal thing I guess, but if I was shooting consistantly 8" to the right I'd consider taking a real good look at arrow spine, I'd think that consistant meant you have good form or it's the same from shot to shot anyway.I'm no expert maybe not what your looking for, apologize if not,Have fun and shoot straight.

I'd recommend a stick-on Hoyt SuperRest, timmed down to fit neatly, and hidden by the soft side of a piece of black Velcro.
Seriously.
I do this on all my bows and never have arrow flight or tuning troubles.
Also, try shooting cock feather inward.

I like all leather plate and rest, or seal skin if you can find some.

+1 on seal skin.

I like the calf hair rest and side plate. I shoot them even after the hair has worn off as just a thin piece of leather and they work great.

I recently went to a rest on my longbow, and don't have any plans to shoot off the shelf anymore.  But when I start getting weird results it is a result of sudden changes to my form, like gripping the bow too tight, over-drawing, or bad release.  Addressing these issues always brings me back.