I really love MERCS, I personally think it's one of Capcom's best games and a worthy sequel to Commando. It really is about time Capcom made a new game in the series!
From the versions on this video my opinions are:
Commodore 64 - Oh dear, brown everywhere combined with a complete lack of detail makes this an incredibly ugly game. The horrible generic music doesn't help matters very much either. The worst of all the versions IMO.
Sinclair Spectrum - Very impressed by this graphically but the sound is just terrible, why is there no music for 128k machines? Especially given that the Speccy has basically the same sound chip as both the Amstrad and ST (which both have music)
Amstrad CPC - This version would be among the most impressive out there if it wasn't for the fact that the play area is the size of a postage stamp. Shame because the graphics and music are both really nice.
Sega Master System - I was incredibly impressed by how accurate this version is compared to the original arcade game. The only downfalls are the weedy sound effects and lack of in-game music.
Atari ST - Quite easily one of the best arcade conversions by US Gold on the Atari ST. It has great graphics, nice music and also retains the simultaneous two-player mode unlike the Mega Drive version.
Commodore Amiga - Another very good 16-bit version. The Amiga version is very interesting because it chooses to match the screen display used by the arcade game rather than a side bar or wider screen.
Sega Mega Drive - I love this game, it's definitely one of my favourite titles on the machine. The best part is the bonus original mode included on the cartridge. If it had a tw-player mode too it would be almost perfect!