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Serperior is pretty ok once you get coil, but it's awful before that, and coil comes very late. Leech seed helps a bit. Shame, it's my favorite of the three and one of my top 3 favorite starters ever.
Emboar is probably the best on its own merits, but darmanitan is legitimately better.
Samurott is nothing special, and you could comfortably replace it with seismitoad or simipour, but it's not like emboar where the other option is leaps and bounds better, they're just a bit better and seismitoad ends up being redundant, since there's absolutely no reason to not use either an excadrill or krookodile in game. Samurott is the best simply because it isn't brutally outclassed or outright bad.
I'd say go for samurott if you just want something straightforward, emboar if you want to power through the early game then power through the late game when you swap it for darmanitan, or serperior if you want a starter that doesn't look terrible.
