There tends to be 3 major roles in fights. There's the support, the ranged/aoe damage dealers, and the melee people who provide mass damage on single targets faster.
Within these base roles the most standout to me are beguiler, samurai and cleric.
Clerics provide incredible support overall. They provide might and massive heals, as well as a group invuln. They can do this also without having to worry about really being killed by anything unless the opposing force decides to focus on him. Even if focused the cleric still can provide himself with 10 seconds or so of invulnerability and still can provide his group mass support.
Samurai has everything a melee would ever need and more. In most cases a majority of damage from anything comes from pure melee damage. Samurai is only outclassed in this area by 2 specs, though it has the potential to easily outdo those 2 if it chooses to. Those 2 specs (runeblade and ninja) both were given a base combat mobility spell each (escapeartist and one). Samurai however was given BOTH of these options to use in combat while also maintaining near equal damage and far higher health and armor. This coupled with the fact their skillset provides incredible choices for every situation (mobility, high damage, anti healing, anti melee) makes samurai the most powerful melee spec by far imo.
Beguilers as a ranged class have pretty much everything they would ever need. The spec that was advertised often as being the master of trickery, using spells to fool the opponent and help control the battlefield has been given most of what was advertised and more in other areas. They were given not only massive amounts of control, but massive amounts of aoe damage which, while being very nuke-wait-nuke oriented, is probably the strongest overall damage of any other ranged spec. They are a class that excels in every field except for survivability due to low mobility and low health/armor. However this weakness tends to be more than made up for by the fact that they have only 1 warmup combat skill that is rarely used, and the tools to prevent anything from doing much damage to them (ranger is an exception).
These 3 specs are all incredibly powerful in 1v1s and tend to be made even more useful in group fights through their various tools and buffs.
The other thing that tends to irritate me about them is that they all work incredibly well together. Cleric/beguiler provides cleric with wisdom making mana not a problem at all. Cleric/samurai provides a team that offers no good target for focusing while giving samurai a might buff. Samurai/beguiler has the ability to lock 1 or 2 enemies down while having the ability to absolutely destroy whatever they target (also might buff for samurai).
I tend to believe in the triangle of beguiler - cleric - samurai. Samurai counters cleric, cleric counters beguiler and beguiler counters samurai. These three specs fight against and with each other to sit atop the other specs overall. There are some strange exceptions such as dreadknight dominating samurai, but overall the three tend to be a bit above the others. The gap isn't extraordinary imo, but it exists.