Of course the best stealth mechs are snipers. Stealth armor works the very best at long range, and is detremental at short range, so mechs like the Pilliger and Archer and Emperor tend to be the best.
Naturaly, the best way to beat them is for some or all of your force to get close, and quickly. I like to send a portion of my force in close and engage thouse stealth mechs that are designed to drop their stealth in order to free heat for their secondaries, which is most but not all of them. If they chouse to do this, then the rest of my force at range can also engage these mechs and bring them down, while if they don't the rest of my force still has to march in to close, but they can't effectively fight off the faster mechs that are on them already.
Another tactic I like, espcealy as the FWL or the Clans, is to take weapons with hyper long ranges, which makes getting into medium range rather easy. The LGR hits medium at 17, ATM HEs at 18, HAGs at 16, etc. Now, you're down from +6 to +3, which is a big difrence, with your opponent still at +4 to hit you. This often works well when only some enemy mechs are stealth, and others are not.