For me one of my favs was a good old anti grav platform with vibro cannon (specially in the days of sustain fire dice) 48" no line of site needed just pick your point on the board draw your line between point and cannon anything touches is hit and the hight av the easier to destroy. Just loved them. Then of course was the d cannon
D-cannon on anti-grav platform. 2nd edition. Not because it always worked like you expected it to, but because it was fun. Displacing a Land Raider with a squad of terminators so it was facing the wrong way around with its weaker back armor towards your guns is... amusing. I actually often used Swooping Hawks as tankbusters back then. Drop down in contact with the rear of the tank. Stuff the exhaust pipes full of krak grenades. Move away. Watch the fireworks.
Ooh yes the Vibro Cannon. I'm kind of sad to see its cool effect gone in sixth, but oh well, time marches on. I've always liked the concept of the Starcannon, but I'm not sure if you can count it as being Anti-Tank, so I'm going to go with the Fire Prism. It's a large blast! It's a focussed blast! It's a lance! It's a strobelight! And so much more!
Haywire! Grenades, blaster, which ever. Second favourite (can't choose just one) is bright/dark lance.
I'm going to say Eldar Revenant Titan. Nothing says anti ... everything like a nice big helping of 4 templates of vitamin D. Nice tank you have there. Now get it off my board.
I quite enjoyed how effective the Singing Spear was against vehicles. Considering it was meant to be some kind of "speshul" weapon, it was quite modest against heavy infantry due to not blocking armour saves. Good weapon!
