So, in alphabetical order:
Darwin / Lifeguard
This will focus on Darwin, but as Lifeguard is so similar, she was included as these should still apply for her, if not more in some cases as she can do it for other people.
First off - Darwin recreated a physical body for himself when he was, to steal a line from Voldermort's lipless mouth - "less than the meanest ghost". This can lead us to (not so much) jump (as fallover) into the conclusion that he/Lifeguard can create a body when there is just an energy force / soul left. Which could mean that death would always be knocked back since a new body could be made. For those who would say this seems ludicrous, a previous adaptation has been to turn into a god. Of death.
He can increase his intelligence, learn languages by looking at them, automatically gain night vision etc.
The downsides to this automatic response is that the body seems to react even if the mind is opposed to the course it takes - his teleporting from the Hulk demonstrating this. It works on everything. He can't get drunk and he'd never be able to claim a sick day.
It can't all be bad though. Just imagine what would happen if he ended up sleeping with a woman who'd been used to bigger . . . guests.
Domino / Longshot
Do you feel lucky? The caveat here is that you cannot just rely on your luck as (for Domino and Longshot respectively) you need to be active in trying to make something happen for the luck to assist it, and if you use it for a selfish or "evil" agenda, it won't work, and may even give you bad luck - this last one is a bit off I feel since as it's their power, anything that keeps them alive over any other act is basically selfish, and evil can be objective. I'm guessing it means according to his own values, but if so, step forward Mastermind or such to rewrite his values . . .
If you knew you'd be lucky though (and for Longshot, luck with the opposite sex is GENETICALLY PROGRAMMED into him) what would you do?
Make money? It's not selfish if you plan to donate it. Have sex? It's not selfish if you're . . . considerate. Get famous? You can broadcast a message of world peace etc.
Money, sex and celebrity seems to be the top 3 of the wishlist, for most people in life, although sex is pretty moot here since Longshot can get it due to his DNA and Domino gets it due to her DNA as well (Damn Nice Ass/ets).
Rather than being a fighter though - although the fact you'd never be hit or hurt, unless you were feeling suicidal or such may remove some of the downside to that - seeing either of them working for a research firm or as a doctor or policeman might be nice. Rescuing people can be done through luck, and their fortune-fields do spread around them. They may even get lucky and develop a cure for the travesty of Man of Bat (cross-canon, but he annoys me. The original anyway).
Elixir
As he can reawaken mutant powers, he can probably even give/create them. Or more usefully make Emma Frost's head actually become canine to suit the bitchy-berater, if not entirely restructure her body into that of an animal - imagine a true Diamond Dog ^_^
Or a female Wolverine. A sex change would be easy enough to manage. Not novel in itself due to X-23, but imagine how Logan would react to it.
Leon Nunez
By tattooing you with a representative symbol, you too can benefit from powers such as smelling rose-fresh, being able to have your friends throw-up or some more beneficial ones such as flight, telepathy or even get yourself a taste of the Phoenix-Force.
This one shouldn't be that hard to imagine the possibilities for - especially if you can steal the thunder of the Phoenix so easily (not that ending up in a coma is easy I guess).
The questions come out when you consider that the powers only come when they're finished. If the plan is to get a set of sleeves, will you need to wait for it all to be done? Does colouring count? And do sleeves only grant you one power or all those drawn? Get on it DC and answer these clearly vital questions!
Madrox
That aside, with the possibility to gather knowledge from a pretty unlimited number of sources - I think it was 40 as his max amount (unless the dupe's dupes can also dupe) - then you can learn everything you need to know quite quickly. Jot down a few key notes and you become a human Cliff-Notes machine.
It also means you'll never need a mirror again, and never lack someone to spot you at the gym, which is quite cool.
Meggan
For such a low-key, back-issue character, I'd say she's quite badass.
Morph
He can fly, he doesn't smell, is telepathic and has the questionable honour of being the first X-man to die.
Honestly his own powers aren't that special, but I wanted to make a Tony Hart pun. So since I've given you a Hart, have a heart, and let this one slide. (Sorry, that was awful o_0)
Prodigy
Scarlet Witch
Siryn
She utilise her powers for echolocation, meaning even if blinded or in the dark she knows where she is. Handy for general radar purposes too. She has sound blasts, flight, yada yada - father's powers. Yet also more. She can create force fields and even control other people.