Yea, GRRM decided to go with a very flatten feudal structure (kinda amusing given the size of Westeros vs IRL medieval kingdoms), but while technically many of the lords under the Lords Paramount are the same rank, they have an extremely varied levels of power and prestige.
Each lordship and family has its own combination of prestige, power, money and ties to the crown that affect how worthy of a match they are - do they claim to have been kings in the past? do they have large navies or armies? are they Valyrian or married Valyrians in the past? etc.
Velaryon are high in this ranking because of their Valyrian descent, dragons and money, despite lacking in other areas.
The Royces, Raynes, Hightowers and Blackwoods all have the prestige and power to be considered worthy over the years.
The Westerlings and Freys are kind of foils in that regard - the former have the prestige but not the power and money, while the latter got the power and money but not the prestige.
When it comes to Lords Paramount specifically it's a bit more complicated - I think that marrying their family can be worthwhile to gather strong allies, even if it can put us in a more tenuous position in the long run. Marrying the Lord Paramount himself is a bigger issue IMO and might outweigh the benefits, especially ones like Jason Lannister.
I can see married to the Tully direct heir working because of their position being more dependent on the crown, as well as the Riverlands being a bit of an extension of the Crownlands, unlike most other kingdoms.
The Baratheons are actually a good match for a Targaryen (hence Rhaenys's mother being one), especially when they are strong, founded by a Targaryen supporter, and are sort-of a Targaryen cadet branch, but I'm not sure there is a god match with one available for Rhaenyra.
Not sure about Forrest Frey though, he got some pros, but I feel like with the whole battle of legitimacy we do need a somewhat more prestigious husband...
