almost anything is good because marths are poor offensively when you are behind them
marth's b-air is awkward/risky/slow/weak(non-tipper), d-air out of shield is risky, potentially too slow, counter out of shield is risky
relatively laggy roll
OK spotdodge
weird gimmicks aside, only real 'safe' option for marth is to wavedash out of shield
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grab is probably the best since marth gets owned out of grabs
aerials are also good, especially good if the marth you face hates being in place and has a tendency to get out of shielding
in fact marth seems so limited/weak when you are behind them that a plain knee on shield --> grab has a better chance of working, very good if you already show ur hand to be pressure-heavy.
i.e. --> you knee shield --> gentlemen everytime, you do it so much people expect gentlemen. so after a knee--> GRABBB
side-b, while not completely safe is also good IMO for same reason
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when ur not behind marth it's much harder to get in, when i do try i either try my best to side-b not like a noob/ hope they n-air, approach with KNEE (worst case is usually a swat away), or dash dance
bob money trained the **** out of me in this matchup, although i still lack experience vs good marths (most marths literally give up vs falcons that they think are gunna **** them up or they themselves suck at the matchup) BUT now I think its objectively an easier matchup than shiek/falco/fox.
Falcon has an advantage on bigger stages, Marth has an advantage on smaller ones, but theres still a lot of room to outplay on both sides. Falcon vs Marth to me is similar to Fox/Falco in that its one of the defining matchups of melee.