I think that stall is very viable as a play style, however there are quite a few threats that need to be dealt with very carefully. It is this reason that a lot of people don't think stall is viable. Pokemon like Breloom have very specific counters that must be used or you lose, simple as. A few pokemon don't even have counters, like Hydreigon or Kyurem-B, and some have counters that don't fit with the rest of the team, for example your counter to Landorus could be Celebi but if you want to use Ferrothorn as your main entry hazard user, you have a problem. For this reason, there will always be at least one common weakness that the stall user will have difficulty playing around. For this reason, most stall teams will look very similar as the same pokemon will be used in order to deal with the biggest threats in the metagame. I don't think that stall is the 'safe' play style any more as it is difficult to play nowadays, but it is still viable and extremely effective when done right.
I have had the most success using stall when basing the team around Hippowdon and Sandslash, therefore my humble opinion is that sandstall is the most viable type of stall. Hippowdon and Sandslash may overlap in terms of typing, but the roles they play are remarkably different; Hippowdon is a wall with various EV spreads depending on whether you want a physical or a mixed wall, while Sandslash is an offensive user of Rapid Spin that doubles as a revenge killer and 'oh shit' button. From there, I added the required teammates to combat the metagame and made sure I included everything a stall team needs (meaning entry hazards and a solid defensive core) and my stall team was made. I don't claim to be the best team builder ever, there are probably flaws in my team that other people will be able to fix, such as my weakness to Sheer Force Landorus, but no team is perfect right?
There are some pokemon that cover a vast number of threats and therefore are extremely good members of stall imo, despite their flaws. I think that Sableye is incredible to the point that I almost prefer to to Jellicent. Prankster is godly, Taunt and Will-o-Wisp are so useful with priority. Recover is also very good on Sableye with priority, it packs an extremely deceptive amount of bulk. A very surprising addition that I find effective is none other than the pimp in suede himself; Weavile. Seriously, it is a better revenge killer and general 'oh shit' button than anything I have ever used. Ice Shard prevents almost everything from getting a clean sweep, especially dragons, Breloom and Landorus. Pursuit removes yet more key threats like Lati@s and the best Rapid Spin user in OU; Starmie. Furthermore, the best answers to Weavile are Scizor, Conkeldurr and Lucario, all of which share similar counters like Jellicent. Seriously, try it out.
Similarly, there are pokemon that seem like good additions to stall but just don't cut it. Firstly, every member of the team needs an instant recovery move; period. Stuff like Landorus-T and Gyarados don't work, they get worn down and eventually removed by the pokemon they are meant to counter. The only exceptions are Ferrothorn as it makes Leech Seed work, especially in rain, while Heatran as it has the typing and bulk to make Leftovers enough, especially with Protect. Forretress is the worst culprit, it is overloaded with jobs and is outclassed at all of them. Choice Scarf Tyranitar is slower than Alakazam and walled by Jellicent after Will-o-Wisp. The Magic Bounce pokemon are frail and weak to Pursuit. The list goes on.
As of the introduction of BW2, stall has been one of the most difficult styles of play to work with. It is also, imo, the most rewarding. Building the team will be extremely frustrating as there is no way to use a team of 6 defensive pokemon and be reliably successful, at least one offensive pokemon must be used as a 'soft check' to a wider variety of threats. Not many teams will introduce anything original to stall, the same pokemon will crop up time and time again. But it can be done, and any player that does will have something to be proud of. Good luck