These comments are entirely my own opinion. After thousands of hours in Firaxis Xcom, I'm seriously not interested in having a jerk-off troll jump in here insisting my opinion is inferior to theirs. Having said that....

IMO, there is no "best" for all conditions in the game. All class usefulness changes with rank progression, equipment availability, mission type, and enemy tiers. For instance, early campaign specials and sharps are pretty bad, but can become very strong mid to late. Early game grenadiers are good, but lose value mid to late until Salvo. Rangers are average early, great mid and situationally great late. The sharpness of the shit/meh/okay/good/great transition lines is less apparent in X2/WOTC than it is in EU/EW, but it's there even so.

There is also a play style factor to be considered. Super aggressive, "in your face" tactics are better suited to some classes than others. While a more cautious approach favors other classes. The bottom line is an experienced player will take advantage of the strengths available to him/her AT THE TIME of any particular mission, at any particular point in a campaign, and take steps to mitigate the weaknesses he/she is stuck with, at that moment.

One method to wrap your mind around this is stacking your barracks with loads of extra troops and trying to run single-class missions (e.g. all sharps, all rangers, etc...) at various points in the campaign. This brings the strengths and weaknesses of each class into sharp focus. It takes a load of patience, but I found it very useful when I took the time to do it.

Good hunting, Commander