BG 1 was a standalone game, and had the expansion TotSC. TotSC just added some sidequest and rised the exp-limit, but had no influence on the main story.
The second game is BG 2, which has the expansion ToB, which both had a decent addition in playtime AND heavily influenced the story, and added so many features it is nowdays considered a third installment despite not running w/o BG 2, thus making the series a "trilogy", where ToB is "BG 3".
BG EE is reloading/reenactment of the original games (the old saves are even compatible?). Basically the same, but can worth the extra credit.
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I'm not familiar with all the new games either, but until you figure it out here are some decently long, and high-quality fleshy games which can take a couple of days to pass, and decently challenging:
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Sky Quest is a top-notch side-scrolling shooter, similar to Raptor on abandonia. I found most skills not-as-handy-as-could-be, but hey, it is at least not fixed at the start, and the gameplay is varying and addicting for sure.
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Epic Battle Fantasy 4 is something like the jrpgs, but man, it is fun and complex and simple enough to play. Epic Difficulty is a real challenge, but playing on normal is already satisfying and interesting (especially if you go for all the medals).