Ultimately, Stoutland. Let’s go to Smogon to see some in-game viability rankings.
Lillipup: A-Tier:
Availability: Early-game (Route 1 from levels 2-4 at a 50% encounter rate).
Typing: The line's members are Normal-types and neutral against everything save Shauntal, whose Ghost-types are immune, and Marshal, who hits the line super effectively.
Stats: The Lillipup line has solid stats except for Special Attack, with Stoutland having 100 Attack, 80 Speed and 85/90/90 bulk.
Movepool: Tackle and Bite carry Lillipup well until Take Down at level 15 and (as a Herdier) Crunch at level 24. Return via TM at Nimbasa City is the line's best STAB attack once they have high friendship, and the Work Up TM can be useful to boost offensive stats.
Major Battles: The Lillipup line has a solid showing in all major battles, as few opponents resist Normal, and Ghost- and the rare Steel-types are handled by Crunch and Dig. Work Up can help the line sweep some fights from Elesa onward.
Additional Comments: Lillipup is consistently a great Pokémon for Gym Leaders but is too reliant on Work Up boosts to do its job at the Pokémon League. Get the Vital Spirit ability as Lillipup, as it turns into Intimidate as a Herdier onward, letting the line take physical hits better.
Summary: Lillipup-Herdier-Stoutland is a solid line, with above average stats, a quite nice movepool, and great typing for most major battles like Gym Leaders.
Now, let’s evaluate Patrat-Watchog (D-Tier):
Availability: Early-game (Route 1, 50%, levels 2-4).
Typing: Normal-type, giving an advantage to Shauntal and average everywhere else save N (where Watchog is mauled).
Stats: Patrat and Watchog have slightly below-average stats aside from decent Speed and Attack (77 and 85, respectively, as Watchog).
Movepool: Tackle upgrades to Retaliate and later Return, with Bite at level 6 and later Crunch at level 16 being staples. Low Kick via move relearner and TM moves like Dig and Grass Knot have sporadic use. Natural Confuse Ray, natural Hypnosis, and Thunder Wave via TM can provide team support.
Major Battles: Patrat and Watchog are decent in most major fights with Normal STAB and Work Up. Late-game, it can sweep Shauntal and Caitlin with +2 Blackglasses Crunch, and can hit N's Carracosta and Ghetsis's Seismitoad with Grass Knot. Low Kick hits the two Bisharp endgame.
Additional Comments: Patrat is very weak, and Watchog is reliant on Work Up boosts to win every major battle successfully. Keen Eye is the preferred ability to avoid accuracy-lowering from a few opponents, although it is not required.
Summary: With subpar stats, a boost-necessary movepool, and general averageness, Watchog would not be the premier choice for a Black playthrough.
Stoutland has a better movepool (therefore type coverage), better overall stats, and better overall usefulness. Use the fluffy bearded doggo!
Hope I helped!
