>>45463781RS = Outstanding, but you'll probably be let down by the conclusion. Plot and characters are a breath of fresh air. Disconnects from RBYGSC plotlines and cast, but throws the odd reference here and there.
FRLG = Average, sorta reconnects RBYGSC through RSE, but the thing is...
E = Bad. Like terribad. Arguably the worst part of the series.
DP = Back in business baby. Another reset, but a blast the whole time. More focus on the cast actually being friends, unlike the previous stories where you just followed a bunch of different trainers doing their own things. Cyrus's portrayal here is amazing, better than any other media.
Platinum = This is honestly mediocre, but you'll read it anyways because it's like an epilogue for DP. It actually does feel like "postgame" in manga form.
HGSS and beyond = All downhill from here, mainly because every plotline gets rushed to keep up with the games. You'll get a good concept at the start of each story, and then no payoff.
tl;dr, RS and DP are amazing, so read both, and you can even skip from RS straight to DP. FRLG is alright, but it'll want to make you read Emerald, which is just...not worth it. Rest of the series is up to you.