As a Pokemon veteran who's owned every main entry and played (most) of the spin-offs I'd really recommend any of the remakes.
FireRed/LeafGreen are remakes of the original for the GBA. Very accessible and has the best add-on content of any of the remakes in the series. The game plays faster and the sprites are very well done compared to the original R/B sprites, which were, pretty ugly 
HeartGold/SoulSilver for the DS is probably the most definitive Pokemon game to date. Two regions, the direct sequel to the originals and the previously mentioned games -as well as having the longest campaign & being more of a remake of Crystal than the original G/S makes it an amazing starting game. It has some of the most beloved features of the entire series such as the Battle Frontier, rematches, GB Sounds, and of course having Pokemon follow you around.
OmegaRuby/AlphaSapphire are the most recent Pokemon games and the most recent remakes. While the content isn't is as much as the last two -it does take everything from X/Y and fix them. The game is very pretty for a 3DS game (imo) and the 3D models for the Pokemon are very well done. The game suffers from having little to no post-game content (well besides the legendary hunting and Delta Episode) but other than that a very solid entry to the series.
I also recommend Emerald. It's like ORAS but with more content -but limited due to being produced on GBA hardware
