Which specs are best and which cant compete then?
It varies between content tiers.
Rogues and Fury Warriors are very strong through pretty much the entire game, although fury can be a pain to gear early on.
Mages are slightly behind Rogues but still strong for most of the game, although there are situational fights in which being ranged is better.
Hunters are strong early on but start to fall behind after BWL, but are great for utility (kiting, pulling etc).
Warlocks are a crapshoot, if classic is maintaining the old limit on debuffs they're going to struggle, but like hunters offer some nice utility.
Retadins aren't going to be topping any meters but may offer some utility.
Shammies are much the same as Paladins, although there may be more utility offered by Enhancement due to improved Windfury Totem.
Priests I can't think of any situation where shadow is going to be worth gearing over a pure DPS such as a mage, there may be some niche.
Druids are much the same as Priests.
Hold in mind that tier sets in vanilla don't really help the case of hybrids either, for the most part hybrid sets focus on healing or tanking aspects of the class, and set bonuses can make a pretty large difference as you progress. Fury is the exception to the rule when it comes to classic, but again they pretty much skip most set items.
I'd say look into the viability of whatever interests you yourself, there's tons of information out there. As I mentioned, I intend to dive into raids and progress, if you want to have a good time leveling up or playing more casually, or intend to focus on PvP it's not going to matter so much. In fairness pretty much any semi-competent group is going to manage to clear Molten Core, it's the later content where things can start to become a bit more rough.