Il keep this short and sweet, the show can be divided up into two sections, seasons 1-3 and 4 - 11.
The actors in both where amazing and their replacements at the start of season 4 where great (but very different in their own right). This show is a comedy filled with one liners that keep you laughing. Though it must be said this is a light hearted comedy, you watch this show to relax and sit, laugh and get gripped. Don't be expecting crude jokes or laughs, expect puns and witty play on words from all the characters, interlaced with practical jokes and hilarious antics.
You can watch pretty much any show you want, most of the episodes are very stand alone, saying that watch them in order because there is still a great story line.
The stand in characters are great, from Flag to Sidney to that Korean black market guy, just adding more humor to the mix. While the main actors are great with their constant banter between them and great laughs.
It must be said the show slightly deteriorated in humor towards the end (10-11), still a great watch but I found myself not laughing as hard at times. However I could not recommend a season to you as each season has its golden episodes, from 5 a clock Charlie, to Henry's Desk one, to any and I mean any of the Sidney ones, to the Korean surrendering to Hawkeye and Honeycutt.
I myself own the MASH box set and find if I am up for a laugh I will just choose a random disk, slot it in and sit back and enjoy, the true essence of light hearted comedy.
A must watch, though do not be expecting the crude nature of Red Dwarf (though I love that show too), think more Friends, in a war zone where not everyone gets along.