What table top game has (in your opinion) the best combat system and why?
I havnt played too many games. Just a bunch of D&D and some GURPS. I can't stand D&D anymore, because I prefer low magic, low fantasy, gritty realism, and tactical combat. I'm sure GURPS seems like it'd be up my alley, but for some reason my group never has fun when we play it.
For the kind of thing you prefer, Riddle of Steel and its successors like Song of Swords are pretty renowned for a fantastic combat system.
Personally my favourite is Legends of the Wulin, but that's a completely different thing.
Try Savage Worlds.
Are there any particular editions you would recommend?
The best in a tactical square medieval game is D&D 4e. But it is high fantasy, high power, high magic, so don't apply to you.
You could go OSR or WH Fantasy.
Mythras might be worth a look. Combat is based on skill rolls, and if you beat your oppent's skill well enough, you get to add an extra effect. These include choosing which hit location to hit, putting your opponent off balance, or finding an opportunity to blind them by throwing sand in their eye. The reach and size of your weapon factor into combat, and some special effects depend on the weapon you're using (e.g. impaling someone on a spear). The combination of hit locations and low hit points makes it easy to take some very nasty injuries, and armour is extremely useful and will probably save your life.
Magic comes in five different types and is really flexible, so it's easy to do low magic, whether that means "magic is rare and diffcult, but powerful", "magic exists, but is very weak", or "magic is a force beyond mortal understanding that serves to create situations for the PCs to resolve".
Not him but there's not much difference between editions when it comes to SW. It's all tactical, highly-mobile combat, although the most recent edition with recent errata is the most mobile.
That sounds excellent. I will look into it for sure.
or the spritual successor of RoS and SoS: Harnmaster
other than that i really, really recommend WFRP 1E/2E
Savage Worlds - Deluxe Explorer's Edition
Also grab any of the Companion books you think you might use.