Conclusions (Calculations Below)
If you know your stats:
12 Sp. Atk - Water Gun			99.77%
11 Sp. Atk - Water Gun			99.67%
14 Attack - Tackle			99.61%
13 Attack - Tackle			90.06%
10 Sp. Atk - Water Gun			49.20%
12 Attack - Tackle			34.34%
9 Sp. Atk - Water Gun			25.44%

Simplified rules:
If 11+ Sp. Atk, Water Gun
If 13+ Attack, Tackle
If 10 Sp. Atk, Water Gun
Otherwise, Tackle

If you don't know your stats:
Water Gun 				64.51%
Tackle*					62.55%

* - This assumes you don't know your stats, but that your stats appear runnable. This is a bit of a paradox and your Tackle odds are actually worse, but some of those missed 2 shots are just unrunnable anyway. In the end, it doesn't matter since Water Gun is better, but I thought this was interesting

=== Data / Calculations ===

"Runnable" Mudkips at Level 5:
- Stat							Odds (all kips)			Odds (no minus attack)
- 12 Attack (0-19 neutral)				42.5%				50.6%
- 13 Attack (20-31 neutral, 0-19 plus)			35.5%				42.26%
- 14 Attack (20-31 plus)				6%				7.14%

- 9 Sp. Atk (0-31 minus)				16%				19.05%
- 10 Sp. Atk (0-19 neutral)				42.5%				41.67%
- 11 Sp. Atk (20-31 neutral, 0-19 plus)			35.5%				33.93%
- 12 Sp. Atk (20-31 plus)				6%				5.36%

Final column is the important Odds for further calcs

===

Poochyena Stats:
- 15 HP (0-29)						93.75%
- 16 HP (30-31)						6.25%

- 6 Defense (0-29 minus)				15%
- 7 Defense (30-31 minus, 0-29 neutral, 0-29 plus)	79.75%
- 8 Defense (30-31 neutral, 30-31 plus)			5.25%

- 5 Sp. Def (0-6 minus)					3.5%
- 6 Sp. Def (7-31 minus, 0-6 neutral, 0-6 plus)		30.88%
- 7 Sp. Def (7-31 neutral, 7-31 plus)			65.63%

==

2HKO Odds (incl crits):

Move Atk/HP/Def			2HKO Odds		Scenario Odds
Tackle 12/*/6			100%			7.59%
Tackle 12/*/7			22.75%			40.35%
Tackle 12/*/8			22.75%			2.66%

Tackle 13/*/6			100%			6.34%
Tackle 13/15/7			93.75%			31.6%
Tackle 13/16/7			64.29%			2.11%
Tackle 13/*/8			22.75%			2.22%

Tackle 14/*/6			100%			1.07%
Tackle 14/*/7			100%			5.7%
Tackle 14/15/8			94.51%			0.35%
Tackle 14/16/8			64.29%			0.02%
--------------------------------------------------------------
Water Gun 9/*/5			99.66%			0.67%
Water Gun 9/*/6			22.75%			5.88%
Water Gun 9/*/7			22.75%			12.5%

Water Gun 10/*/5		100%			1.46%
Water Gun 10/*/6		99.66%			12.86%
Water Gun 10/*/7		22.75%			27.34%

Water Gun 11/*/5		100%			1.19%
Water Gun 11/*/6		99.66%			10.48%
Water Gun 11/*/7		99.66%			22.27%

Water Gun 12/*/5		100%			0.19%
Water Gun 12/*/6		100%			1.65%
Water Gun 12/*/7		99.66%			3.52%