I tend to value a click as being worth 2 credits or 2 cards. It's the exchange that shows up in a lot of cards. So 1 card = 1 credit is probably a fair exchange value for analysis.
In any case, I think you're right about HB having the best, most straightforward, ability. You install something (which is almost every turn) and you get a credit. Not a lot to think about there.
As the card set grows, Weyland has the potential for more, especially since the ability triggers *every* time a Transaction Operation is played, not just the first. I imagine we'll get new versions of Day/Night Shift with the Transaction keyword and that'll double the number of cards that trigger the ability and make it more palatable. But even in the best case, it requires drawing out the deck. Definitely the weakest ability by far. You'll only get 3 extra credits from this on average I imagine. It'd be better if it triggered off of Transaction abilities as well.
NBN's is a bit harder to quantify. It's not just the credits spent breaking the trace but also the credits you cost the Runner to match and avoid the trace. So the value is potentially doubled. If NBN spends both credits on traces during 5 different turns and the Runner pays to match you on at least two of them, you're already on parity with HB's ability at 14. Of course, are they credits you would've need to spend had you not had them? There's also the value of those credits helping to land a tag on the Runner. So it's a powerful ability, probably the most valuable if aggressive gameplay.
Jinteki has the nebulous additional value of costing the runner opportunities from the random discards and the outside chance of killing them if they access too many agenda at once. It does force the Runner to waste a couple clicks keeping more cards in their hands than they normally would. All in all, I think you're right about how valuable the power is. It'll do 6 net damage at the very most (potentially more with different agenda in the future), in practice probably 4-5. Four cards plus, let's say two clicks wasted to cautious play, so a value of 8. A little bit better than Weyland at least.
Weyland and Jinteki's abilities have a hard cap while NBN and HB continue to pay out even if the game drags on. (Well, technically, HB is limited by the number of non-Operation cards in your deck but that's not likely to be a concern.) So the long game favors their abilities.
The Runner abilities seem to all be stronger than Weyland and Jinteki's.
Gabriel pays out 2 credits the first time you hit HQ each turn and has cards geared towards him hitting HQ as a normal course of business. And it keeps paying and paying as the game goes on. Probably the best with focused play.
Kate has a similar ability to HB that's not quite as strong since it's just hardware and programs. But she also gets a permanent link bonus and cards geared towards programs and hardware. It's a strong straightforward ability that you're always going to benefit from.
Noise's virus mill… Weak if you don't play to the ability, strong if you're aggressive about laying waste to R&D. It would probably mill 4-5 cards in the course of a game with standard gameplay but if you're focused on taking advantage of it, then it's going to get costly for the corp. There's added value from making agenda easier to steal because they're now in archives and out of reach for any corp except HB. It also triggers every time a virus is installed, not just the first time one is. Probably *the* ability that's going to become more and more powerful as new cards come out. There's likely a deck centered around decking the corp to be had here a year from now.