The benefits of flanking in RON are larger than they appear.
There is not only the insane 100% bonus for side hits, but there's the simple positional effect of crossing the T: having numerous units concentrate fire on a single one.
For long thin lines, archers are the best flankers. Large numbers of them will concentrate on one unit at a time, and if set to aggressive, will then move up to target them one at a time, tearing them absolutely to shreds. Because of their speed advantage, horse archers are the best kind of archers to use.
For thick lines, heavy cavalry supported by cavalry archers are the best to use; this is because with a thicker flank frontage, the cavalry can attack more units simultaneously while still retaining the flank bonus; against a long thin line, horses will spill over to the front and rear of the line, costing time and not receiving full flank bonus.
Because of the fact that archers are the most likely units to remain in a strict line formation on the battlefield, it's possible that slingers/javelins may actually be the best kind of units for flanking, because they will get flanking bonus on units that they actually counter. Does the 100% flanking bonus apply before or after the countering bonus? If after, it's not merely huge but is POSITIVELY MONSTROUS.
Optimal formation for flanking from the side (ie approaching from the side) is line abreast.
Approaching from the front, in a meeting engagement, the best formation is probably reverse wedge, because it delivers flanking shots on a unit approaching frontally. Alternatively, you might use the left or right echelon formations for this. The reason you don't use V for attacking from the side is because you would in fact be attacking some units from the front and the back, and therefore would again not receive the flank bonus.
In the strategy guide, they said that they felt that forward wedge was the best formation for defending as archers. This may well be true, but I think that given the unit AI, it may be good for attacking as well... position your units so that only the very point can hit the enemy formation, and if they're set on defense or aggressive, they'll tend to come and attack in a broken-up fashion. If they are a large melee formation, they'll be broken up and disorganized, because they'll be shuffling over themselves trying to reach the point unit, when only a few of them can actually attack at the same time.
However, I don't think this should apply to attacks on an archer formation, because they'll simply cut down the point units one by one as they come in range. Archer units should definitely be flanked, and preferably by counters.
I'm always all about the tactical game, and it's been implemented better in RON than ever before. It's where most of these other games, notably EE, fail miserably.
By the way, does anyone remember the Mongol formation-splitting tactic from AOK:TC whereby you would recursively split your formation into smaller and smaller groups to kill enemy knights if the opponent wasn't paying attention? Any clues on how to do such a thing here?