hard to tell....but I like the last one. Removing the seat, if it has a quick release, is pretty common in the US for mountain bikes. Other options include the bolt mentioned above and when locking running the lock cable through the seat also.

off topic My friend in the cube next to me is moving to Hyderbad in a couple of months (he is originally from bangalore) to manage business applications at our growing office.... If you know Oracle....pm me your cv.