QUOTE(SEMSEO @ Jul 30 2008, 03:25 AM)
That is a good question.
Can the most searched keywords be regarded as the toughest to SEO ?
or
Can keywords with the most content be regarded as the toughest to SEO?
Like Randy said, neither of them have an effect on what's harder to rank for.
QUOTE
It would be good to know which industry is the most challenging to do SEO so that I could take up this challenge. It would be good for my resume or SEO portfolio.
Once you're taking on competitive niches like mentioned above there is money to be made if you get rankings for money making phrases and i doubt if you get a taste of that you'll be working for clients or an employer.
But then again you need obtain success to get that far.
What looks good for your resume/portfolio since you're not interested to benefit from your own SEO efforts directly, is to show how you've helped businesses increase their traffic from keywords that convert to the action the business in question desires. So that the traffic has a noticeable revenue impact.
That is what will make an impression for your future employer/clients.
Know what you're doing otherwise don't even do that, you want to help these people you know and not helping them hurting their business.
So only do it if you're confident enough about your SEO experience.
And i don't think you're experienced enough to take that step, because of the questions you brought forward.
Get your own sites up and make some money, if you want a more steady income then work with clients or for an employer.
If you are wondering why I asked about porn industry, the answer is NO, I am not an SEO for porn industry. 
I asked because I saw a lot of spamming done by porn sites.
