My worst AI exploit is in Age of Wonders: Shadow Magic BUT there's a justification...sorta.

Flying units (even the really weak Zephyr Hawks) can stymie a stack of Shadow Demon Lords, Ballistas, Cannons, or the best damn (non-flying) critters in the game by sitting in a tree. Seriously.

The Ai can't "see" a flying unit that is directly on top of a tree, so all you have to do is stay out of the initial range of any ranged units and you can 'end turn' until turn 26 when the tactical battle automatically ends. This was an exploit that you could use to block enemy entry into your cities, but a community mod patch changed it so that flying units are blocked from the space over doors, so you have to use an alternate: put the flying unit on your wizard tower or other building - even if the city is overrun, you'll retain the city as long as the flying unit lives to turn 26.

Here's the explanation. On the final scenario of the campaign, you are scattered and under siege by tons of shadow demons who WILL route you and your allies in no time flat - it is a nigh-unwinnable situation without that exploit. With it, however, you can keep your allies around to trade spells and resources, act as buffers, and generally have a fighting chance until you're ready to take on the All-Devourer.

My worst strategy was champ flooding people with The Goths in AOE2, noobs always expected you to loose since Goths had weak walls (and noobs always like to turtle). If they let me get into the forth age I can produce 4 Champs per second, usually from barracks just outside of their main town.

Champs couldn't hold up to archer or Chok fire for very long but I could produce replacements faster than they died and my economy could more than pay for it. If they somehow had a strong force of something by then (like Jag Warriors or a lot of Choks) I could pump out some counter units briefly (well I needed a Stables for Knights so it was 50/50 whether I'd have any close by stables up as backups).

Normally I'd end the game at the start of the third age, or even in the second if I pulled off an early attack on their economy.