All the monkeys have the same stats.
First real attacks [Vine Whip | Incinerate | Water Gun] Panpour's attack is strongest.
Second Set [Leech Seed | Yawn | Water Sport] Hard to judge. Leech Seed is nice but Yawn can go a long way as well.
Third Set [Seed Bomb | Flame Burst | Scald] Seed Bomb and Scald are the same power wise, but Scald lowers attack through Burning. Flameburst can make up for the 10 Point difference in Double and Triple battles with splash damage.
Last move set [Grass Knot | Fire Blast | Brine] Fire Blast is better right off. Grass Knot depends on opponent, and Brine really shines if and only if you get them past 50% health.
All three have their merits, but all three are easily outclassed early in the game. You can replace Pansear with Darumaka and later Larvesta if you feel the need. Panpour is outclassed by Tympole's line which you get just a little later. Pansage is outclassed by both Petilil and Cottonee which are just after the second badge.
None are worth keeping past the third badge, nor did I keep mine (Pansear). I actually found my Purloin to be better than the monkey. They were cool and all, but not worth the effort to keep raising.