FR/LG are attempts at re-making the early games, so if you've already played those I don't see why you'd want to start with effectively the same things. Not that much has changed, they aren't separate games per se. You could, of course, start with the second generation if you want a quick start, as the beginning becomes increasingly more laborious as time goes on, but that may depend. Whichever you prefer, playing through the different ones would seem slightly disorderly, as you don't particularly have any clear idea which is good or if you'd want to continue past a certain point, and if you're just getting back into it rather than having any particular enthusiasm for the series then you might recommend looking into which games seem most suitable, rather than presupposing a linear development in series of Pokémon games.
Anyway, the third gen are perhaps a decent combination of accessibility or shiny impressive things and gameplay for such as you, but can perhaps seem a bit gaudy for a beginning, the second gen might be best for players more interested in the series, the fourth gen is a bit slow and perhaps a bit 'animated' for new players (eg. 'Starly' and 'Bidoof' replacing 'Pidgey' and 'Rattata,' although perhaps Starly could be of some trivial interest if you stuck with it for a bit), but I mean if you wanted to start those out you might notice some of the newer Pokémon, and in that sense you might want to choose carefully. Newer generations, apart from perhaps in the Johto games, which is to say G/S, do tend to be a bit light on earlier Pokémon and might not help with that, so if for some reason you wished to use them as a sort of Safari for surveying Pokémon, you might want to keep that in mind. I mean, someone saying something like, 'I used to like the FF games [eg. the Hunger Games but with Missingno in charge of such things], then stopped, now I'm going to play each of them in order,' might come across as a bit more akin to a fan, so you might not want to go in that enthusiastically if you do. Presumably you might have a reason for playing them, and want to play the ones which suit this, but the reasons for your interest are at this point fairly unclear.
Other way you could go by is going from gen1 onwards
That particular order seems a bit problematic, as most of that time is spent on spin-offs or re-makes, and if they wished to get an impression of any particular gen of games, they might want to focus on the actual core, for instance Gold/Silver, Ruby/Sapphire, etc., rather than games which are basically just 'Gold/Silver with more focus on Suicune,' 'Ruby/Sapphire with a different plot-line focusing on Rayquaza,' re-treads, etc. Presumably you don't experience one gen by experiencing a different one.
