Mongoose's Runequest is a good system with a one caveat: as a GM, you're not afraid to adjudicate on the spot. There are some grey areas in the rules (read: it's not as codified as 3E). It can be a blessing for some GMs, a curse for others.
If you like the freedom bestowed upon you by the system, if you like to house-rule and make the system your own, if you like rather intuitive, rules-light systems, this is for you.
RuneQuest Deluxe combines the base rules, the Companion, and the Monster supplement. You've got all you need to start right away.