Well, few favorites exist due to my low amount of 'completed' battles, so instead I'll give you a mini-history and point out my one or two 'favorites.'
(36 battles and counting--not counting my older ones)
Oct 1st, 2009:
NLBFT 13 R4: Metal Man vs. Acradius - Non-starter
Aug 29th 2009:
NLBFT 13 R3: Metal Man vs. Trickster again - Surfiiiiing... down an elevator...
June 23rd 2009:
NLBFT 13 R2: Metal Man vs. IRHP Again - Coke machine of death!
Aug 15 2009:
Metal Man vs. VGA Again - Non-starter
July 30th 2009:
Metal Man vs. Goofball/VGA June 5th 2009:
NLBFT 13 R1: Metal Man vs. Guildmaster again May 23rd, 2009:
Metal Man vs. Vapor again April 18th, 2009:
Metal Man vs. Weegee March 10th 2009:
Metal Man vs. Seraph, third time March 6th 2009:
Metal Man vs. IRHP - Mostly complete
Feb 23rd 2009:
Metal Man vs. Speed Feb 2nd 2009:
Metal Man vs. Sabrilocke Oct 18th 2008:
Metal Man vs. Vapor again - Non-starter
Sept 15th 2008:
Open Fight vs. Inferno Aug 30th 2008:
Metal Man vs. 1-up yet again Aug 28th 2008:
Metal Man vs. Trickster - Another mostly complete one
July 21st 2008:
Metal Man vs. Guildmaster July 8th, 2008
Metal Man vs. Inferno Dragon Again June 4th, 2008:
Metal Man vs. 1-up Salesman - Weird
April 2nd 2008:
Metal Man vs. Joker - One of my better ones
March 18th 2008:
Metal Man vs. Vapor - More or less complete
March 12th 2008:
Metal Man vs. Repster March 3rd 2008:
Metal Man vs. Inferno Dragon - Complete battle
Nov 9th 2007:
Free-for-all (I used Metal Sonic here) Oct 9th 2007:
Metal Man vs. Galefore July 13th 2007:
NLBFT 11 Round 3: Metal Man vs. Selene Starblade June 23rd 2007:
NLBFT 11 Round 2: Metal Man vs. Scripture May 31st 2007:
NLBFT 11 Round 1: Metal Man vs. Deathscythe Jan 8th, 2007:
Metal Man vs. TehDarkness - Pseudo-Nonstarter
Sept 7th, 2006:
Metal Man vs. Seraph II - Also a non-starter
June 12th, 2006:
Free for All in Station Square (Fake Sega Battlefield) - Relic from when I got Shane to revive Sega Battlefield for 10 seconds
March 10th 2006:
Metal Man vs. X-3 vs. Vengeance - Weird long-lasting free-for-all
Jan 14th 2006:
Metal Man vs. Zora - Non-starter
Sept 24 2005:
Metal Man vs. Seraph - Non-starter
Feb 23 2005:
Metal Man vs. X-3