For day-to-day interacting with Twitter, you can’t beat a good third-party app in my experience.

As to which is the best one, that’s largely a matter of subjective opinion although some popularity measurement is good, too.

My favourite? TweetDeck followed closely by Seesmic Desktop which I’m trying out. Those are what I use on the desktop and the laptop. I have other favourites for mobile devices.

But if you look around, you’ll see that there are literally hundreds of applications you can use to interact with Twitter, nearly all for free.

Here’s as good a popularity measure as any – the top 20 Twitter clients (third-party apps) as measured by numbers of users by Twitstat. The numbers update daily.

I’ve taken the top 20 you see above from Twitstat’s larger total (some 224 different Twitter clients, says Twitstat) from which you can note some interesting aspects.

First, the ‘app’ out front by a large margin is the web, ie, direct access to twitter.com with a desktop (or laptop) web browser, at 26.61% of users according to Twitstat.

In second place is TweetDeck with 11.22% of users. Twhirl (the still-active predecessor to Seesmic Desktop) is fifth with 3.69%. Seesmic Desktop just makes it into this list at number 20 with 0.78%.

While TweetDeck is the current leading desktop app, I reckon Seesmic Desktop will soon come knocking on its door especially as that app continues developing to add new features and functionality and as Twhirl presumably gets less developer attention. See what Seesmic founder and CEO Loic Le Meur has to say.

But doesn’t that view assume TweetDeck does nothing more? I guess it does, which of course I just don’t see happening especially as it’s widely known that developer Iain Dodsworth – who secured venture funding in January – is developing a Pro version, aimed at the business user with additional features for which you’d pay.

And I’ve not even mentioned how both apps also integrate with others services such as Facebook, 12seconds.tv, seesmic video, and more.

If you want to see blow-by-blow comparisons and other commentary about these competing apps, you have plenty of reviews to choose from.

These three apps – TweetDeck, Twhirl and Seesmic Desktop – are all desktop apps: you install and run them on your computer. Each uses Adobe AIR, meaning among other things that they’re cross-platform and will run on Windows, Mac and Linux machines.

What about mobile?

Twitstat’s numbers show that the most popular app for using Twitter on a mobile device is Tweetie (at number three with 6.25%) which runs only on the iPhone; a version to run on the desktop, on Mac computers, is in the works.

I use Tweetie on my iPhone and it is excellent. On my Nokia N95 8GB, I use Twibble Mobile (not in Twitstat’s list although its desktop sibling Twibble is, at number 46 with 0.21%) although I’m also trying out Gravity (number 32 with 0.36%). Both terrific apps.

And there you have the interesting issue – the market for Twitter apps is not only huge, it’s hugely fragmented right now.

As soon as someone develops the killer app that contains that magical balance of features and functionality and who knows what else that causes the “OMG, I absolutely have got to have that!”, they’ll clean up.

I wonder who that will be.