There are many factors to consider when selecting a pool and reward system (in what I consider decreasing order of importance):

• Pool operator trustworthiness (absolutely critical!)
• Pool hopping (applicable to reward systems with a proportional element only)
• Stales and server reliability
• Fees (including withdrawal fees)
• Variance
• Bonuses (find our next block for us and get a bonus 1 BTC!)
• Block reward (50 BTC or 50 BTC + transaction fees - does not apply to PPS)

Pretty much you just have to work out what's best for you.

PPS reduces (but does not eliminate) the need to trust the pool operator and completely kills variance but will usually come with a significant fee.

Solo mining eliminates all problems on this list apart from the rather extreme variance (this is one of my favourite methods).

At this point the best returns would probably come from pool hopping on the largest proportional pool you can (switching to a low fee pps pool).  This will usually give a high expected return and at low variance.  You have to keep your eye on the mining forums though because it's only a matter of time before proportional pools die out.

The worst would probably be mining on a proportional pool without actually hopping but this could be trumped by paying a 10% fee at a PPS pool.

If you have an ethical objection to pool hopping then it's hard to beat solo for expected returns.  Perhaps PPLNS on mineco.in has the highest expected return without the extreme variance of solo mining.

Personally I mine PPLNS at simplecoin.us (I lose block reward transaction fees but gain more in reduced stales).