What games that are meant to be RPGs (with all that implies) have the best rules for tactical combat?
My metric for that is based on how much fun the combat rules are to play by themselves, with the RP side of things minimized. I'm not particularly concerned with realism or simulation value or such things, nor is balance between PC effectiveness a consideration here.
Going by that, I rate the DAMMIT, SNIPED! Lancer rules set from Massif very highly, as it's become my go-to mecha combat game, edging out Mekton Zeta, various Dream Pod 9 offerings and Battletech/Mechwarrior by a fair margin. If you've only used the game for roleplay, try taking a couple of small (3-4 mechs) teams each with however many License Levels you like (more takes longer, obviously) and playing it out on grid map or as a minis game sometime. Works surprisingly well for something that was designed to battle NPCs with their rather different rules.
Unsurprisingly, I'm also a fan of D&D 4e (late era, when the math fix was in) and ORCUS, although 13th Age doesn't quite come up to snuff - but it's much stronger as an RPG IMO, so there's that. Takes more effort to set up a scenario as game-unto-itself than Lancer, though.
For something simpler but with fewer meaningful choices, The Fantasy Trip is a nostalgic favorite for me, although I'd be even happier with something that fell between TFT and GURPS. Melee and Wizard do all the work of paring out the RPG bits for you, and you can crib scenario ideas from the solo modules, so that's nice.
I know a few people who still swear by SPI's ancient DragonQuest RPG, which had its own combat board game box released as Arena of Blood. Not sure I get the appeal personally, but it still has enough fans it merits mentioning, at least. It's a product of its era and a bit clunky even for that, but it works okay for gladiator fights.
Come to think of it, Savage Worlds definitely deserve to be on the list too. I'll still happily play Great Rail Wars (a dedicated Deadlands-setting minis game that's pretty much proto-SW) any chance I can, and the combat rules only got better from there.