I think the noticeable answer to that is Google, because as Google page 1 accounts for 50% of ALL web traffic for any niche or keyword, then any service who is NOT on page 1 in Google for 'top SEO services' is clearly not a top SEO service. Surgeon, first heal thyself and all that.
The next step is to further identify your selection criteria. We've already decided that they need to be on page 1 of Google, but what else? I think there are a number of things you should think about and spend some time thinking about, including:
Experience - Have you ever thought about how many years experience they have? How long have they been active and online? Only those who have been around a while beyond doubt know the cycles and what to watch out for and then do when the icebergs next approach.
What sort of guarantee do they offer? Do they offer any at all? Is it wishy washy? Do they guarantee page 1, even 1 listings on Google? Do you get your money back if they fail? Do they only charge when they succeed? Many will say they can not guarantee because they can not control Google. Without labouring the point, top SEO services do not have that worry.
Keyword Research :-
Do they mention the importance of keyword research? If they just spout off about how many directory links or social bookmarks they will do each month then they and you, will be missing a fundamental point. In fact, maybe the most central one of all. Keyword research is a extremely important and advantageous way of getting more web traffic and more importantly the right traffic.
Can you get access to the person in charge? Quickly? When things are not happening, you need answers and quick. Is the main 'man' available to you at a moment's notice? Or do you have to wait days for a response from an email or response on a forum.
Are they too cheap? Keep in mind you pay for what you get! Buyer watch out. There's a lot of ingenuousness out there and pricing too low is always a pretty good indicative sign of that ingenuousness. If they don't charge enough then it normally means a too heavy reliance on automation or even shoddier, they will end up not doing what they said they were going to do.
In the end, when looking for and then choosing top SEO services, don't be terrified to ask questions and DEMAND results. If they can't provide satisfactory guarantees of those results then I suggest there will be better alternatives out there.