It's likely that whoever holds Harrenhal is the most powerful secondary riverlands house, not the Freys.

[thelittledragonthatcould]

Seat of a king.

Extensive, fertile lands.

They held the tournament of the era.

The Freys have a bridge.

Your first and third points are irrelevant.

We have no idea if the Whents have larger, more fertile lands. We know the Freys have villages sworn to them as far down as the Blue Fork such as Hags Mire, that is a huge area of influence.

[Beorn Snow]

At that point doesn't Harenhall belong to Bolton/Hoat and therefore not count among "Riverrun's bannerman?"

I mean, TC's point wasn't that the Freys were weak, just weaker than the holder of Harenhall

Short answer is no. The holder of Harrenhal remained a Tully bannerman. Just as the dreadfort holder is a Stark bannerman. Feudal systems don´t really exclude multiple loyalties anyway? (Did Robb ever give Bolton Harrenhal? Wasn´t he just using it as a fort anyway?)

[Winterfell is Burning]

We don't know which side they fought on, Hoster was married to a Whent so they may have sides with Robert.

Unlikely, considering that Oswell Whent and his brother seemed close to Rhaegar, and the Tullys didn't even went to the Tournament of Harrenhal, which seems to indicate there were problems even before the war.

[thelittledragonthatcould]

Unlikely, considering that Oswell Whent and his brother seemed close to Rhaegar, and the Tullys didn't even went to the Tournament of Harrenhal, which seems to indicate there were problems even before the war.

We are told that the Mootens, Darrys, Goodbrooks and Rygers all fought for the King. We are never told that the Whnets did.

Considering that Minisa Whent was married to Hoster and that Lysa and Cat, who both are half Whents, are betrothed to Arryn and a Stark it is likely that they were conflicted in their loyalty.

[Frey Pie]

I would think they are pretty close in power

I would put the riverlands houses like this:

Freys- largest army, richest

Owner of Harrenhal- most prestigious

Mallisters- bit of everything. Rich, prestigious, militarily strong, very good commanders etc

While the Freys may be the richest and most populous, I would think many houses would prefer to make a match with the Mallisters or whents

[Victor227]

There's a mention in SSMs that Tywin thought that Amory Lorch and Vargo Hoat would be able to hold Harrenhal against Roose's army with just a few hundred men (and would have had a good shot had Vargo not turned from within the castle during the Weasel Soup incident). Perhaps you'd need a few thousand defenders to hold off the 'million strong armies' that the walls were built to repel, but with a garrison the size of Winterfell's you could be about as effective. Lady Whent must have had some serious issues, like being abandoned to a man, or overestimated Tywin's army, or simply she didn't have any fight in her.

So, Harrenhal isn't just a pile of dusty ruins, it's still a powerful fortification. The fortunes of Harrenhal seem to follow its masters. It was a great seat under the Strongs, and was responsible for the greatest tournament in a generation under the late Lord Whent, but under Lady Whent it declined into a mere ruin with no authority over the surrounding area, and I'd imagine its done so before. It commands the entire region around the God's Eye, which is rich and fertile, and there's a number of lesser lords and landed knights as well.

In a sense, it seems much more dependent on its holder than most seats of power. A strong lord makes it strong, a weak one makes it weak. That's certainly happened before in other regions, like when Tytos Lannister had next to no power in the west because he demonstrated he would never use it. In the case of Harrenhal though, due to the (relatively) fast declines of the houses that have held it, it just seems much more glaring.

[aryagonnakill#2]

No 1 has mentioned the fact that the lords of Maidenpool are very powerful. They have the largest town, that even borders on a city.

[new djinn]

We don't know which side they fought on, Hoster was married to a Whent so they may have sides with Robert.

Hoster's captain of the guards was Robyn Ryger. That didn't stop the Rygers from being Targ loyalists, and considering the whole HH tournament as a maneuver to arrange a meeting between Rhaegar and Great Lords, that puts the Whents clearly closer to Rhaegar than Robert & Hoster. It seems that in the Riverlands, the closer you were to KL, the more dragonish you were.