I have owned a 2004 950 adventure, a 2012 990 adventure R and a 2014 1190 adventure. It all depends on what you want to do with the bike - mostly road touring, gravel and some off road, mostly off road, etc.
- the 950 is carbeurated and they are extremely reliable once properly sorted - I got 59,000km out of mine and it's still going. My friend has 96,000km on his and it reads perfect compression when tested last month.
- the 990 is more powerful (rated at 20hp more but it feels like a lot more), is fuel injected and can feel like it's got a snatchy throttle from factory but that's a very easy and cheap fix. I loved my former 990R, best adventure bike I ever had. Both the 950 and 990 are equal for off road riding in terms of weight, capability and components - the 990 was an evolution of the 950.
- the 1190 is a completely different animal in every sense of the word in terms of power, complexity, cost to buy and run and it's off road capability and durability.
depending on what you want, how you want to ride it and how much you can spend, I'd say:
1- 990 adventure model years 2009 and up (worthwhile updates in 2009) - best for touring, city hooliganism and highly capable off road with the right tires
2-
2007-2009 950 super enduro if you could actually find one - lighter than its adventure counterpart by over 60 pounds, zero wind protection and smaller fuel capacity so it's less touring-friendly but incredible off road: the best geometry, weight balance and suspension of any large off road bike ever made
whatever you find, feel free to ping me for any input - I know these bikes inside out and I know most of the ones out there in the ADV community. There are some rare gems worth looking for if you can budget extra for them: any 950S (taller suspension), any 990 Dakar or R, 2013 Baja edition. I would not hesitate to buy a bike with 40,000km on it provided it's been properly maintained and updated (updates are widely documented and cheap to perform). The LC8 engine and platform is virtually bulletproof. They were designed to compete and endur true Dakar conditions without failure. They really are that good.