Tropius would definitely be one of them. Even though I hate bananas I could eat them for sustenance and its leaves would provide valuable cover from the sun. It's also very tall at 6'7 so I could climb it and use the height to my advantage.
My second choice would have to be Suicune. Are legendary pokemon even allowed? But either way, Suicune has the ability to purify water - something very important - and produce wind, which while not necessary, would make my life stranded on an island much more enjoyable.
Last would be Lucario. Besides the fact that it's strong and would be able to defend me from anything that might hurt me on the island, it could help me collect materials for a fire and such things like that. It also learns valuable moves like Heal Pulse which would be important if I got injured somehow.