Which DKC was the best? I keep hearing great things about them.
And I already know that the music alone is [tornado fang]ing orgasmic.
I've grown up with these games. So I'll give the really long, detailed explanations.
DKC 1 is the only one where Donkey Kong himself is playable. Its sort of a mellow game, music is quieter, the only real gimmick stages are the ones that are 80% Barrel Blasting, Stop & Go Station, Blackout Basement and the awesome Minecart stages. Its also insane fuckballs hard especially in the Ice stages and the Factory levels (of which there aren't enough of, the Factory levels are the coolest). Animal Buddies are there but nobody but Rambi and Enguarde ever come back after this game (Squawks doesn't count in this game, he's a light in one of the cave stages, that's pretty much it.) Secrets in this game can be obscure as [parasitic bomb], leaps of faith and whatnot, hell one bonus stage is hidden... in ANOTHER BONUS STAGE, getting 101% doesn't do anything special either. Its a great game, but lesser to the other ones.
DKC2 ups the difficulty a bit in some places, adds new stuff that would become common in the later games, coins to buy stuff (though in DKC2 they're only used for hints, saving more than once in a world, using Funky the first time in a world, or the trivia game to earn extra lives), Bonus levels that reward you with a different kind of coin that's used to enter the secret world (that upon beating earns you a different ending) and DK coins which in this game are basically bragging rights to claim that you are better than Mario, Yoshi and Link. It also introduces Dixie Kong (Who is the best character) and has some of the best music in the series (Stickerbrush Symphony, anybody?). Its a simply amazing game.
DKC3 is notable in that both Donkey Kong AND Diddy Kong are absent, making it the only one without a character from the first game playable. There's a whole new cast of bear characters to interact with, a bunch more [parasitic bomb] to collect (But not DK64 tier yet.) with Banana Birds, DK coins, Bonus coins, Bear Coins, items you earn from bosses to get upgraded boats so you can progress to new worlds, it also has two endings (one for beating the final boss, and another for beating the boss of the Lost World AND collecting all the DK coins for the final boat AND collecting all of the Banana Birds) and a total of 105% to get (which is only possible by playing on TUFST difficulty by using a cheat code, which removes midpoint barrels). The game has a lot of gimmick stages though (Sawblade levels, Minecart stages BUT IN THE SEWER TUBES INSTEAD, a race against time stage, like five levels of just using an animal buddy, Rocket Run, etc.) and possibly the WORST animal stage in "Purple Squawks adventure through pipes with barrels and bees because he can't shoot nuts like the normal squawks can because purple squawks is a [tornado fang]ing loser". Also rambi isn't in it, instead you get Ellie the Elephant who sounds like a diseased pig when she sees rats or gets hurt, And there's also Perry the Parallel Bird, who basically exists as a reward for you if you manage to get him to the No animal sign without smacking him into a [tornado fang]ing bee. Its not without its issues (I mean, this is Rare in the middle of the 90s, three years before DK64, there's signs of that insanity already showing in this.) but I personally love the game anyway.
I say get all of them, but if you only get one get 2.
Can't really comment on the DKL games since I never played them when I was younger, but they're basically a bit messy, trying to emulate the style of the games on a handheld with 4 shades of gray/olive green doesn't really translate well.
DKL1 has some strange [parasitic bomb] though, apparently DK goes to the city, he gets bombs to blow up boulders in the paths between levels, he collects coins with his face on them, there are warp rings from Sonic, and flying pigs are enemies in addition to actual DKC enemies.
ALSO, forgot to mention this but buy any of the DKC/DKL games on the eShop and you can get a 30% discount on Tropical Freeze/Returns 3D respectively (also if you buy a DKC on the Wii U and you have your NNID linked to your 3DS you can get the discount for Returns 3D that way too and I imagine vice versa for DKL and TF.)