ink saver (main) is an absolute no brainer. besides increasing the amount of times you can flick before depleting (from 6 to 7), it also increases the amount of ink you can flick from a depleted tank and you're desperate to fight someone back. It's one of the most important abilities for Dynamo probably. Its little cousin, Ink refill, is not as useful because the difference is marginal, but you can still make use of it.
Cold-blooded is really good and is my personal favorite on shirts, but you don't technically need it if the rank/group of people you play in never uses echolocation. Alternatively you can make use of ninja squid, but Dynamos are not thaaat hugely invested in close ranged sneakiness and the swim speed penalty is pretty cruel, so take that as you will.
Ink resist is HUGELY important on dynamo. Every single bit of mobility you can retain is crucial. Do not wear any shoes that don't have ink resist. You will have a worse time without it.
Damage ups are the next big thing, as they allow you to increase the effective range at which you more reliably OHKO without RNG screwing you over (explanation is a little bit long on that one, I can elaborate if you wish).
Tenacity is pretty good in itself, I don't think it's a necessity on the Dynamo though. Your vanilla special charge rate far surpasses what Tenacity usually give you and if you're playing good you'll mostly prevent your teammates' deaths anyway. Can still be a good asset if you're the carry train anyway.
Special Duration Up is trash on Dynamos. On the normal Dynamo you extend the duration of Echolocation for maybe very few seconds which you will barely ever need (you either already killed the people concerning you or they fled to safety), and that's not accounting for the people who use Cold Blooded anyway. On the Gold Dynamo, it's literally worthless because it doesn't affect the Inkstrike in any way except it gives you more time to choose a location (which you will never need). Don't bother with that.
There's also more abilities that a Dynamo slightly benefits from more than others that I listed in my
absurdly long Dynamo guide, but overall it's pretty much just flavor and personal preference and likely doesn't influence the outcome of your match all that much. Only the above give you a serious edge.