Jalorda has the Defenses and Speed, Embuoh has the best Attack and HP, while Daikenki is sorta a balance among everything.
Jalorda (Serperior) has a very shallow movepool, so you'll probably need to find Hidden Power and get some coverage on him. If not, STAB, Dual Screens, and Leech Seed do great in-game, especially against the Elite Four!
Embuoh (Emboar) is slow, fairly bulky, and pretty strong. He learns many recoil moves, and, thanks to his HP, can be used as a good suicide sweeper. Head Smash, Take Down, Flare Blitz, and Boiling Water will make a near-unstoppable Embuoh!
Daikenki (Samurott) has very good coverage attacks, and, while he doesn't seem to have a niche, unlike the other two, it's the most balanced choice, thanksto well-rounded stats, and potent attacks to boot. Boiling Water, Megahorn, Ice Beam, and Dragon Tail will prove to be a versatile in-game set that lets you control the battlefield!
Really, you can't pick a "best", with starters this generation, because all stats between the three have an equal total. Just choose which one you like best. :)