Minimum mountain groupset hard enough to resist abuse I'm looking to upgrade my mountain bike and I would like to get something better than I have, if justified. I've never ridden any groupset better than alivio/deore, which work ok for me and have tolerated some abuse. I'm riding harder these days and I would like to know if a better groupset would be more durable. I'm not very concerned with the "bling" factor or the weight, just something that will stand some "serious" trail. Any thoughts? Please feel free to elaborate on the brand or isolated parts if you want.

Edit: I would like to make the question more specific, because my intention is not to start a discussion about Shimano or SRAM. I assume that both brands are ok. Let's see: I want the bike to do cross country but not downhill. When I say that I ride "harder" I mean that I've gone from following a trail to now jump, go over rocks, some mud, a bit of water, etc ... That has made my drivetrain to get out of tune and drop the chain frequently, even I if adjust it before every ride. I am discarding here my shifting technique as a source of problems because I know how to shift correctly. I thus want to know at which level of the groupset hierarchies (no matter if it is Shimano or SRAM) this is less frequent. That is, that you adjust the gears and they stay adjusted for more than a ride even after some abuse. If I am going to need to adjust the gears every time no matter if the group is a XTR or a Acera, well, then I will rather save my money. But if XTR is stronger and stays tuned for more time, then I don't mind paying for the quality. I would like to hear from experienced riders if they noticed (or didn't) a great improvement in reliability (not weight or coolness-factor) when they bought a more expensive groupset. I hope that the question is clearer now. 

Thanks!
Note: this is mostly based on my personal experience. Its an
  incredibly subjective subject, so there is no right answer. Keep in
  mind that in reality one level up or down will be completely
  unnoticeable performance difference to most riders.


XT is considered the "Sweet spot" for performance, weight and durability.  Probably X9 in the SRAM range. XTR and X0 are for sponsored riders, rich competitive riders and those that have to be seen to have the best because they will never be...

Alivo is definitely entry level and although it starts out OK, is less durable and quickly becomes unreliable if used hard. Deore and X7 are known for good durability - not up to  XT in quality and feel and durability - but not far behind. IMHO Deore and X7 are a good price point for a budget conscious weekend warrior - especially if you can get last years stuff at a good price. 

So depending on your budget, I would recommend looking towards Deore  or SRAM X7 level gear. From you post, I suspect XT is too good for the bike and you won't get the advantage of the extra $$$.

So where is you problem? The bike "was good once" - so the level of the gear is not the problem. I suspect the gear on it is worn out..... 

I would start with a close look at the chain lines - check the Bottom bracket for movement, and check the derailleur hanger for alignment (LBS has a tool for this). Check the cables are moving freely - I suggest replace the rear derailleur if you have any doubt or its more than a year old. All this is low or no cost and can make an enormous difference if they are out. 

Assuming these are OK, I would start with a closer look at the chain, cluster and chain rings. Especially check the chain is clean and free running, with no tight links. If these have not been replaced recently, its the first thing to consider. At the same time, check the derailleur for movement - this is hard if you are not experienced. A little is OK, a lot not - and how to tell - got the LBS and look at some new bikes, look at some mates bikes. 

Here is where you can start throwing money into a seemingly bottom less pit, so be careful. Don't feel the need to replace with higher spec components; don't go up more than 1, at a stretch 2 levels - so Alivio to Deore is OK, Alivio to XT - think very carefully. Keep it all about the same. 

You are likely to find that parts prices mean an upgrade of bike is worth considering. A MTB is more than the drive line - tires, shocks, frame geometry all make a bigger difference than smooth gear changes.