NEW:
inkzooka is really useful as the only way you're going to take out defensive snipers who don't allow you to sneak up (since splattershots have that quick charge that rivals yours in speed and e-liters have burst bombs or kraken).
however, it doesn't come around often enough on squiffer and the inkmine sub is pretty much just occasionally splat assistance (it rarely gets a kill on its own).
still, it's useful on long stages that highly benefit chargers as it's the ONLY squiffer that has the capability of range. it's also not bad in rainmaker where the inkmine can block off routes and cause confusion (though i still prefer other squiffs in this mode) and for some reason i have luck with it in SZ (this is probably where i get the most usage out of it).
CLASSIC:
this is the safest squiffer, and pretty soliid, consistent weapon. i use a lot of point sensors to check enemy ink and and around corners, to help out or notify teammates, and to improve my accuracy. it's main drawbacks are that it only has one offensive ability (the gun itself) and players tend to rush you when you pull out bubbler. i tend to force myself to bubble quickly when around teammates to make sure it gets used, even if it's just intimidation and doesn't get any splats (the bubble comes quickly after all, so you might as well use it). i also have gotten quite good at nabbing risky squids at point blank when they try to rush my bubble.
it can be used in any mode and any stage, but it doesn't really excel at any of them? i guess TC is pretty good with it since you can bubble on the tower (while still having access to your main weapon, unlike the kraken) or tag tower-riding squids with point sensors so it's easy to take them out quickly. it's a middle-of-the-line squiffer for me -- but its consistency is what makes it successful.
FRESH:
obviously the most aggressive squiffer, and my personal favorite. i might have preferred splat bombs to suction since they use less ink, explode faster (really important when making a getaway), and tend to be trickier... but whatever, suction bombs are powerful. kraken doesn't come that often and enemies will push you the F away so it's not always successful (and even gets you killed), but it can help you make big advances (kraken to just around a corner and then leave a suction bomb trap for when they follow) and is well worth spending the effort to stealth behind enemies to release it at just the right time.
it's useful in almost every mode... worst is probably SZ since kraken doesn't ink and people tend to know when you're coming in this mode, but it's biggest drawback is kraken + grates. i generally don't play fresh on hammerhead ... ever. it also REALLY depends on my mood and mental state how well i can outmaneuver my opponents, far more than with the classic which is always comfortable.
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tl;dr i prefer the fresh squiffer for aggression and versatility in the stages it works on, but if there are poor stages or i'm having an off day i still use the other squiffers.
as for abilities, i use tenacity, run speed up, ink res up, and ink recovery up (this is a placeholder, though, for run speed up so i can free my tshirt slot). i also have some superfluous quick respawn (the only two hats i wear both have triple quick respawn :/). i also seem to have a LOT of success running haunt in solo queue even though the damage up bonus is wasted on squiffer.