Heavyweight has Junior Dos Santos as a clearcut contender
Light Heavyweight has Rampage and Rashad about tied
Middleweight has Okami as the clearcut contender
Welterweight has Nick Diaz as the clearcut contender
Lightweight has such an ambiguous contender pool, that's it's kind of subjective. The weird thing, is you have a few worthy guys who have not really beaten guys who were much near the top 10 or were at the bottom of it.
If you ask many people, a lot would say Jim Miller, some would say Melvin Guillard, some would say Dennis Siver is close, and a few would declare that Clay Guida is it.
If only Melendez was brought over, because he'd clearly be the one.
I would say as far as which fighters are closest proximity, it's as follows for the UFC pool of LWs:
1. Jim Miller
2. Clay Guida
3. Melvin Guillard
I have to give Guida the edge for beating Gomi and Pettis which I think is slightly better than Roller and Dunham.
On the other hand, was Florian really as bad as some see him following his loss to Maynard? Despite losing to Maynard and Penn, Florian had no other losses in over 4 years. Florian made Guida look fairly outclassed in the standup and now Guida is more highly regarded. It's funny how times change. Even Siver is more regarded than a few years ago and has pulled off some suprising wins and improved a great deal. It seems that the LW title clusterfuck/logjam has made some weird winning scenarios where guys like Guillard and Siver have been able to shine whereas Guillard especially was regarded pretty low a few years ago. Maybe moving to Greg Jacksons really helped him that extra amount.
I like your top three. I definitely agree. Miller deserves way more respect. 
I think it's because so many of them have a similar "ultimate wrestle-boxer w/ a little BJJ who bounces and circles around the cage a lot" style
the gameplans are redundant but effective against the right stylistic match-ups
and on top of that a lot of those guys don't finish many fights wich leaves things a little hazy and non clear-cut
The Notorious OMG - I think it's because so many of them have a similar "ultimate wrestle-boxer w/ a little BJJ who bounces and circles around the cage a lot" style
the gameplans are redundant but effective against the right stylistic match-ups
and on top of that a lot of those guys don't finish many fights wich leaves things a little hazy and non clear-cut
Miller finished his last two opponents, Guillard finished 3 of his last 4 (all by 1st round KO), and Guida has finished 3 of 4 (not finishing Pettis is no shame).
They all qualify for a title shot. Miller especially does is he convincingly beats Henderson.
- Miller
2. Guillard
3. Not guida, he never impresses me against top 10 opponents (not that it should matter if he gets enough wins, but it doesnt make me want to move him up the list).
Other than that inb4 someone says Maynard since technically he is still a contender.
Guida beat the guys (Pettis) who was going to get the next title shot. The shot should be Guida's/ The Pettis win is bigger than beating Kamal Sharlous and Shane Roller.
GUIDA
It's a bad time to be a LW, that's for sure.
Guys like Miller, in any other weight class, would have already had their shots like 6-12 months ago. It's just the nature of that division at the moment though, very congested at the top.