So, I've basically "beaten" the game at this point (i.e. constant clear of all stages) and my personal take is:
Saw = primarily useful for close to doubling gems/money/points and cuts HP of all balls in half by splitting them one time (won't split multiples). You have to place them at the very top of most levels, positioned where most balls that fall through will hit it.
Magnet = best end-game for damage dealing. Likely even best early/mid-game if you can get its dark power to sub-5 seconds quick.
Bomb = Best finisher for when magnet gets everything else low enough damage. - Acid = Speeds up the time-table on when bombs become useful. For example, instead of 20-25 iterations of magnet, it might only take 15-20, speeding up death on balls by, say 5 seconds or so.
Guns = primarily useful for the first couple maps so you don't have to waste precious other towers too early.
Shrink/Electric = no use whatsoever. Maybe "just for fun" supplemental damage in early stages or levels just to toy around.
Personally, I'd prioritize magnet over saw first. Yes, it's nice to get that doubling earlier on, but a full-dark magnet will have a greater point-gain impact than a full-dark saw. The gems will also come faster because you'll have more maps open with more gem drops per map.