Okay, well... if you're looking for a serious amount of gameplay...
Then FireRed or LeafGreen are the way to go. They are remakes of the 1st generation games with 3rd generation mechanics. The original map has been updated and a new region is available to explore. This is for GBA.
SoulSilver or HeartGold are also good choices for the DS. I would suggest buying them used if you want them to be cheap however, as the games normally retail for 40 dollars as a result of the Pokewalker accessory. The games have been updated to 4th gen mechanics and everything about them is way more pretty. A few things have been redesigned, and new things such as a new route, Safari zone and story events are also present.
If you're looking to get current on Pokemon, I highly suggest getting Black or White version. They are essentially an entirely different experience far removed from everything else in the series. The story is actually interesting (at least for a Pokemon game), and half of the massive region doesn't open up until the post game, leaving you with tons to do long after you finish the storyline. Additionally, no old Pokemon show up at all until the post game, meaning its a completely fresh experience wherein you are playing blind, like everyone did the very first time we ever played Pokemon.
Those are my recommendations for the main series. There are a few other games out there that are fun though. The Pokemon Ranger games for DS are rather enjoyable, but far different from a standard Pokemon experience. And both the gamecube games, Colosseum and XD: Gale of Darkness have complete storylines and interesting battling mechanics.
I-I really hope that I helped..!