YOU CHEATER!!! Just kidding. If you wanna cheat, then here are some good codes to start off with.

AAVETLGE One Hit Knockdown (doesn't work always)
SZVALPAX Infinite Stamina
ALNEVPEY Never lose stars you gained
GZKETGST Never lose hearts
KVKAAGLA Refresh Stamina between rounds
(don't have to press Select)
AAVETLGA Normal punches do more damage
STNAPUIV Stunned punches do more damage
SZVAAOIV Take less damage from punches
SZSELPAX No energy replenishment for opponent
(barely works)

Here are some good and weird codes submitted by Daniel T. Some work great, some are quite glitchy. I think you will enjoy them either way.

YZLOSS Instant win!
UZLOSS Instant loss!
TZLOSS Instant controversial TKO code!!
KGLOSS Mario keeps on saying "Fight!"
APGENP Stars are more common
KOAEEA Begin with very little energy
SIZEKO Round ends when one of Mac's punches is blocked
SNAKEO One seriously messed up screen!
STNAPUIV Stunned punches do more damage
PGAAPS Only Mac's energy bar runs down
PGAAPG Mac snaps! Maximum point outburst. (use the "a" button when punching,
it will work better. Give Glass Joe a punch to the chest to see Mac at his best.)
IPAKIN Major sound mess up
EATEAT Tyson absolutely chewed up the game!! Opponents change shapes and sizes when punched.
OKPGOK Messes up screen and opponents. Look at Tyson's eyes!
KUZEAK Always win by split decision without fighting! See new ending vs Tyson! (game freezes at some point)
OKLOSS The controversial KO code!! After you knock down an opponent Mario will start the count, then as the opponent is getting up another Mario will enter and continue the count up to 10 for the cheap KO. (If Mario freezes at a certain number, press the reset button and try it again.)
ANAZON Game can still be heard and played, however screen is blank.
YOZEAK The Classic code!! Watch the entire Mike Tyson's Punch-Out!! Movie in black and white.
YATZEY Mike Tyson's Black-Out!! Are you afraid of the dark?
ANIELT No time! Some of the sound is messed up
NAPANA Both boxers begin with no energy!
OZLOSS Always lose by controversial KO! without fighting
AXIONS The TKO code! After an opponent has been knocked down wait a while until Mario's twin appears and announces the TKO victory.
APIONS Mac stays in white clothing!
KOAUTO Opponents change colour and nearly all can be easily KO'd. Mac cannot regain hearts as fast and cannot get up if knocked down. There are no messages between rounds. King Hipo is the only opponent who is impregnable and unbeatable!!
GKAUTO Almost the same as the KOAUTO code except that the opponents do not change colour and in between rounds there are messages, but Glass Joe is saying all of the quotes. King Hipo is still invisible, however the fight can now go on for the full 3 rounds where he can be beaten by decision. Opponents get up more frequently.
OSTILL The illusion fighters!
LAYZEY Mario and his twin switch between referee duties
NAYZEY Marios have a white streak on their hat
SAYZEY Marios wear a white hat
PXPXPX removes opening Mike Tyson screen
KOOUTO Changes opponent's fighting pattern
KKOUTO A must see against Glass Joe and Tyson as they both dodge attacks, something they never do!
KOOLYO Harmless opponents (this can be good or bad)
TAYEIT Changes rounds, Tyson's punches are not as powerful
EANALL Phantom punch knockdown! (a cool variation of the one hit knockdown code)
TONILL Too close for comfort
TATALL Mario's counting is extremely fast, as well as Mac's victory celebration
EXSELL One hit knockdown (does not work on all boxers)
ZAPEEE Mac begins with no energy
SAPLLL Many opponents such as Great Tiger stay down for the count after being knocked down. Others such as Glass Joe get up with more energy and opponents such as Tyson get up with less energy.
ZAPLLL Similar to the SAPLLL code, however some opponents get back up with no energy. Tyson can be KO'd in the second round.
AAPLLL Similar to the SAPLLL code with a few differences and sometimes Tyson can be KO'd in the first round.
OAPLLL Similar to the SAPLLL code with a few changes and Tyson can be KO'd in the third round.
GIGEEE Mac's energy does not run down, however he can still be harmed.
IOUSSS Opponents get back up with less energy
GTTZZZ Darker arcade style colour
ATTZZZ A bit of a darker colour than in the GGTZZZ code
STTZZZ Classic colour!
PTTZZZ Gameboy colour!