my favorite tabletop system is white wolf's world of darkness D10 system. D20 system would be a second place assuming you could find supplements that suit you.
I like the flexibility afforded to the person running the game (storyteller) and while it's tactical enough, it doesn't become a tabletop game like 4th ed D&D.
I am going to give my World of Darknesss core book another read to see how easy it would be to use this for near future Sci-Fi. I love the d10 system for the same reasons you mentioned. Its fast, tactical and can be very narrative. I hate having to use a grid when running a tabletop RPG and never do, so that was the reason that 4th ed DnD and myself never got along.
From my experience with the D20 system it doesn't work too well with technology. In the past I ran a Dragon Star campaign that was a mixed magic and technology system using the D20 system. It was really enjoyable at lower levels because of the deadliness of the technology. A group of Kobolds which got its hands on some plasma rifles could decimate the group in the first round with a well coordinated attack. At later levels because of the way the D20 system levels heros the technology became less realistic, with the warriors being able to take multiple plasma rifle shots to the face and laugh it off.
I tend to like systems where as you increase levels you become more skillful, but that doesn't mean you become able to take more damage.
TSR's Alternity system does this well. Your durability, the amount of stun damage, wound damage and mortal damage, is determined at level 1 and never increases. That means that at level 20 you die as easy as at level 1, but at level 20 you are more skillful and would be able to take out multiple foes in the same time it takes for a level 1 person to dispatch one enemy.
A downside with the Alternity system could be the amount of rules needed to achieve the realism, which bogs down the combat system with roles to hit, then armor absorption depending on the type of hit, and then only damage application.
At the moment my idea is for a short campaign based in the near future, a Blade Runner/Total Recall/Prometheus type setting. So I want to find a good system to bring across some realism without totally bogging down the action with complex rules.