Depends how much money you're willing to spend, and what you're after out of it.
The top Valve package, the Valve complete pacakge, comes with every Valve game based on the Half-Life engine and also every Half-Life 2 engine based Valve game (Source Engine).
If you're just wanting it for single player, in the complete package, you're getting Half-Life, Opposing Force and Blueshift on the Half-Life engine side, and Half-Life 2 and Half-Life 2: Episode One on the Source side.
Multiplayer wise, it depends on what you enjoy, you'll probably find all of the older Half-Life engine games quite poor in terms of graphics etc (Counter-Stike 1.6, Counter-Strike: Condition Zero, Day of Defeat, Team Fortress Classic, Death Match Classic and Ricochet).
If you go for just the Source Premier Pack, you'll get Half-Life: Source, which is the original Half-Life converted using the Source engine. The advantage here is you don't have to buy the Half-Life 1 based games from the Valve complete pack to play Half-Life: Source, but you would be missing out on the great add on packs Opposing Force and Blueshift.
You've got the complete pack costing you around £50, and the Source Premier Pack costing you around £36. You could get some of this cheaper by getting a mix of retail copies and registering them all to one account, but it can be difficult with older Half-Life games due to them being limited stock and CD Keys being already registered a lot of the time.
If you are a patient person, I would almost suggest waiting until Half-Life 2: Episode 2 pre-order becomes available, as that will likely be made part of the main Valve complete package, with Team Fortress 2 and Portal. There'll likely be a 10% discount, however it'll be a bit more expensive due to the number of new titles being added to the bundle.