Game and Watch has problems in the next matchups:
Marth: almost every stage, except for RC maybe
Snake: Mostly in neutrals, like FD or SV, counterpicks help
Diddy: Same as Snake
MK: every stage, i dont know if there is a stage in which this matchup is better (better=not that bad)... maybe Halberd?
Toon Link: Same as Snake and Diddy; though this matchup isn't really impossible
DK: very questionable, this matchup is somewhat between 45-55 and 55-45
Possible Solutions:
Against Marth: MK, Snake, King Dedede. Other options: ROB, Wolf, DK
Against Snake: MK, King Dedede, Olimar. Other options: Wario (though Wario is better on counterpicks), ROB, ZSS, Pikachu
Against Diddy (on neutrals): Snake, Falco, Marth, ROB, Lucario, Toon Link, Pit, Peach, Luigi. Other options: MK, Wario, Olimar, Sonic, Mario, Wolf, Pokémon Trainer (Squirtle), Yoshi, Kirby
Against MK: lol (Ice Climbers on neutrals). Other options (on neutrals like FD or SV): Diddy, Snake, Falco, Wario
Based on these matchups (the worsts), some viable characters for secondaries are:
MK: Ties with himself, ties or have small advantage over Snake and Diddy in neutrals, and wins over Marth (though in neutrals is a little harder). He also defeats TLink and DK
Snake: Ties with himself, small disadvantage against MK, defeats Marth and Diddy. He also defeats TLink, and has a very small advantage over DK
King Dedede: Beaten by MK and Diddy, wins over Snake and Marth, he also has an advantage over TLink and ***** DK
Falco: beaten by Snake and Marth...
ROB: Badly beaten by MK, and has no clear advantages over Snake, Marth or Diddy
Wolf: ties with Snake, Marth and Diddy. Beaten by MK
Ice Climbers: Along with MK he ties with MK on neutrals. Loses to Snake, and has small disadvantage against Marth and Diddy (though neutrals are IC's friends)
Wario: small disadvantage against MK, loses against Marth, and ties or has small advantage with Snake and Diddy, though he is better in counterpicks than neutrals
Olimar: Best counter against Snake, though he loses against Marth and MK, and ties with Diddy
Conclusions:
MK is the best secondary
If you dont want to go the MK way, i'd say the best secondaries are:
Snake is a good option. You'll have problems against MK (which is always bad) and you'll have to be better on dittos. You'll alsohave problems against Marth on counterpicks, either you go G&W or Snake
With King Dedede you'll have problems against MK everywhere, and Diddy on neutrals (though not that hard). Marth on counterpicks could be troublesome
Ice Climbers: You'll have problems with MK on counterpicks (Though G&W is better against MK on counterpicks than in neutrals). The big advantage with Ice Climbers is that they "solve" your bad matchups on neutrals. Marth and Diddy will be a lot better in these stages. Problems: Snake on neutrals and Marth on counterpicks. Also IC is maybe the hardest (or longest) character to master
Wario shines in counterpicks, where you'll most likely use G&W; he can do better against MK, Snake and Diddy in general, though against Marth on neutrals you'll be doomed
Besides MK there is no clear option. My pick would be King Dedede or Ice Climbers. Wario can be a very interesting option if there are no Marths around
