Star Grunt II is a generic SiFi rules set, available for free from Ground Zero Games. They do provide their own background and models (a hard SiFi setting) which I also quite like, but the rules can be used for basically anything. There are even fan-made versions for other periods.
The game has a very elegant system, where you do not get modifiers to dice, but the dice themselves are modified, and the dice type is also the base for ranges of that unit. For example, a regular unit has a quality die of d8. When they fire their weapons up to 8" (one range band, the range band of a green unit with a quality d6 is 6", etc.), they roll the d8 and a firepower die based on their weaponry. The target rolls a d4 (fixed for the first range band). When the enemy is in cover, the d4 is raised to a d6. When the target is 20" away (in the third range band of the firing unit), the d4 gets raised to a d8, because of the cover to a d10. The range band differs by unit quality, so better quality units can lay down effective fire at greater ranges.
The result then is determined by the number of dice the attacker scores over the result of the defender's die; 2 successes (can cause casualties), 1 success (suppression only), or no success = no effect. Even if you roll a d12 (the highest possible die) you can still roll a one – so the higher the die type the better your chances, but nothing every is guaranteed. If you roll a 1 on your defending d12 against a 2 and 3 on the attacker's 2d4 you are still doomed.
This general principle applies more or less to all mechanis.
The game has rules for infantry, vehicles, fire support (mortars, artillery, orbital bombardment), minefields, electronic warfare… all of which fit into the game nicely and don't feel bolted on.
Hit models are marked casualty, but their exact state is unknown. A unit can take a reorganize action, in which casualties can be treated: they might just be shocked and return to the fight, turn out to be wounded, or to be dead.
A unit is in one of five confidence levels (confident to routed) which determines what actions it can take. Taking casualties etc. forces confident test, where your confidence can drop. A leader can rally a unit and raise its confidence level again. Shaken units for example will not advance towards enemies. Now we come back to the casualties: In a confidence test, each untreated casualty provides a negative modifier. Leaving wounded behind is an even greater negative modifier. So unlike in many other wargames, hit models are not just taken off the board (except when dead), they need to be carried around – which gets difficult when there are more wounded in a unit than able bodied troopers. And while this is quite detailed, the mechanics behind are actually quite simple.
Another great aspect are the rules for snipers. They can be added to squads, acting as designated marksmen, bolstering the firepower of the squad, or deployed as individuals (or small two man teams) – this is where it gets interesting. Snipers on their own have larger range bands, and always cause at least a suppression result (psychologic effect of sniper fire) even if they miss. They are deployed with three counters, one real one and two dummy counters, all within 6". So the enemy knows the rough location of the sniper, but not its exact firing position. A sniper can "move" between the counters, thus changing his position without the enemy knowing where he is – he might also have stayed in place. To effectively fight a sniper, it first needs to be detected.
The command level on the table is also an interesting aspect. A command level can, on their activation, activate a unit under their command – even one that was already activated, when they pass a communications test. When you play large games, this can cascade: company commander activates two platoon commanders, the two platton commanders each activate two squads… a powerful mechanic that adds much to the game's tactical depth.
From a pure rules perspective, this is my favorite SiFi rule set.