The first film is pretty long and drawn out. It's main failing is that some bigwig at Paramount at the time thought, we spent the most money on the effects, so the effects get a lot more screentime than needed. The scene of Kirk appoaching the ENterpise in the shuttlecraft is worth watching though.
ST2, The Wrath of Khan is probably still the best TOS movie ever. I recommend you watch the episode "Space Seed" again before watching ST2. Khan's motives are better understood.
STIII is pretty melodramatic and it starts right where ST2 left off.
ST4 is probably the funniest ST movie, even out of ALL the ST movies, and probably the best money maker (adjusted for inflation). Kirk and Co. in the mid-80s is a pretty hilarious premise.
STV is WIlliam Shatner's directorial debut, it's also the only movie in the ST franchise that didn't employ the effects company Industrial Light and Magic (they were on strike). It's pretty much the cheesiest of the TOS movies.
ST6 was directed by the same guy who directed ST2 (Nicholas Meyer), so it's a dark and nitty gritty. It's the darkest film of the ToS movies, and is second to Wrath of Khan in terms of plot quality (maybe even better). There's political intrigue ^.~!
You know, I'd almost recommend you skip the first ToS movie. You won't miss any important character development.