You'll get a variety of answers as to which is best, as you see from the other posts.
I wanted to add a couple of points not mentioned above.
The Chakra amps will use a different active card than the others. It won't fit in the other amps, and the cards for the other amps won't fit in the Chakra amps. So make sure you get the right card for the amp you purchase. It will also be a little bit more expensive since it's a newer version. The cards come up for sale fairly regularly, although not every day, so you should find some without too much trouble.
The LK 140 is better than the other LKs because it has more power. However, when you go active, you don't need as powerful an amp because all the power is going to only one driver in the speaker, bypassing the crossover. The crossover is where you lose most of the power. Switching from passive to active with the same amp is like having an amp upgrade as well as a speaker upgrade! The less expensive LK 85 would do quite nicely.
I've had LK 85s, LK 100s and 5125s. I like the 5125. I think that the switching power supply makes it that much more efficient. My preference, maybe not somebody else's.
Although the Klout is a superior amp (I believe Ivor Tiefunbrun himself had Klouts in his personal system until recently), I think that given the cost, even of a used Klout, I would just save my money for a Chakra.
If you want the best bang for the buck, go for an LK 85 if used actively, LK 140 if used passively, or a 5125 for either active or passive use but you want to have that fifth channel for the future. If cost is not a problem, then get a Chakra.