I'd want to be a Bothan. I was one in Star Wars Galaxies. Check it out!

http://en.wikipedia.org/wiki/Bothan#Bothan

^^When I first read your post I thought it said Botham(as in Ian), and I thought 'WTF? Who wants to be Ian Botham?'.

Although his 'tash is rather alluring.

A Lion, Why? because of this: "during a mating bout, which could last several days, the couple copulates twenty to forty times a day"
- http://en.wikipedia.org/wiki/Lions#Reproduction_and_life_cycle

I'd be a dog. Licking my unmentionables is way too hard now. I only succeed 1/3 of the time. Also, having sex with a bitch and getting stuck together, until we finish? Hell yeah!!

Liger.  It's pretty much my favorite animal.

Biggest cat in the world, too.

Kree!

too bad they are not born natural in the wild and they cant repreduce

do the agent clones from crackdown count as a species?

Siberian tiger or king eagle or royal eagle or whatever that subspecies of eagle is called in english.

As long as I'm adopted into a caring, non-abusive family I'd pick a dog. Probably a Golden Retriever like my dog. 

Such simple lives of eating, sleeping, shitting, playing, begging for human food, etc.