I've been thinking about which rulesets to use for Valiesk and have reached a conclusion.
My first choice, for platoon-level engagements, is Fast and Dirty v4.0 (FAD4) by Ivan Sorensen. This is a free ruleset available on the 'Net, which is quite interesting and dynamic when it comes to infantry tactics and morale, all while staying quite simple to learn and use. It was designed with sci-fi in mind, but could definitely be used for any modern or sci-fi milieu starting from WWII and up to futuristic sci-fi.
This is a platoon- or company-level ruleset, which, while having relatively cumbersome vehicle rules, is great nonetheless for infantry engagements with a few vehicles added for good measure. And it supports a wide range of weapons, equipment, and troops qualities.
The second ruleset I intend to use is USE ME Modern Warfare by 15mm.co.uk. These are very elegant rules, using two characteristics per until (ELAN, which is unit quality, and Move, which is its speed) and only 1d6 for all rolls. It is fast and furious, and highly suitable to quick resolution of small-units engagements, such as Russian Spetznaz rescuing hostages from an embassy taken over by Black Freedom, or the Valieskan Police raiding a Vory warehouse.
This is a modern ruleset based on a previous sci-fi ruleset and produced by the wonderful 15mm.co.uk. Oh, and I was its co-writer, so let's say I'm a bit biased towards it. It was fun statting all the real-world forces in USE ME stats!