For close combat, I'd say Brushes and the Carbon Roller are the best melee ambush weapons, while the Aerospray and Splash-O-Matic are the best ranged ambush weapons.
The Splattershots/Hero/Octo are all good for midrange ambush due to their relatively high damage-per-hit and decent fire rate.
For insane snipers and gunners, the Squiffers, Bamboozler, and Mini Splatling are best Charger-type ambush weapons. Remember to doubletap with the Bamboozler!
For Subweapons play, clutch Splat and Suction Bombs can produce some of the most surprising ambush-OHKOs. Seekers are too noisy for that, but can still be handy for surprises.