Personally, I think Splinter Cell is the best out of all of these with no competition, but the thing to know is that Splinter Cell is so specific in its content that you either hate it or it's you favorite game so if you really liked Conviction it might be worth getting some of the previous ones. On the other hand Conviction is totally different from the first four of the series, it resembles more of an Assassin's Creed feel to the game by all of the techniques you use (both are made by Ubisoft). If you do like the feel of hiding in the shadows and the whole espionage set up then you should most definitely look into the first four. Here's their order 1st to 4th and how I feel they are:
Splinter Cell: Stealth Action Redifined
Truly one of the best stealth games out there, in my opinion the most interesting and challenging of the series, and it was really the only competition for Metal Gear Solid back in the day.
Splinter Cell: Pandora Tomorrow
It's not too bad, the set up is kinda boring though, and the gameplay didn't really change from the first one, there's more guerilla warfare, and hiding in bushes, tall grass, etc. 2nd hardest of the series, and a disappointing ending.
Splinter Cell: Chaos Theory
Probably the most bought of the series, and probably the easiest too. the moves and variety of taking people out has greatly increased (thanks to Sam Fisher's knife) and it's much easier to toy around with guards. This was a pretty good game, and has great online play (Spies vs. Mercs).
Splinter Cell: Double Agent
By far the worst game of the series depending on which system you buy it for. The first version was for the 360 and PS3 and it was awful, but the 2nd version for the xbox, ps2, and gamecube was surprisingly brilliant, and got a really good rating, possibly the best in the series to some people if you get the second version.