So... Yeah, I have evil fragmentation from hell on my 250 GB HDD that's almost full and I would really, really like to defrag it now. So, what's the best way to do it without it taking whole day?

I use MyDefrag. I don't know how good it is compared to others, but it works well.

Everyone always says Diskkeeper, but I don't think it's free (maybe they have a free version). There's a million free defrag tools, I just can't vouch for any of them.

I use Defraggler from the same people who make CCleaner. Which I also recommend.

So, how does Microsoft's default defragger compare with other tools?

In my experience slowly.

How about the results? Are they comparable?

It's difficult to benchmark a defrag. It gets the job done certainly but I can't tell you how effective it is compared to a third party solution. If you have big hard drives like me though you can expect the MS defrag to take most of a day.

Well, I have a 1TB drive, but since the computer I used before my current one had a 70GB hard drive and around 20GB of that was the OS, I've learned to be very space conservative. The only heavy duty storage I've got going currently is all of the install files and extras for my GOG games.

I am using PerfectDisk.

Seconded.

Depends on the version of windows, the Vista/7 Defrag is... well I don't want to use the word 'quick' but its not the all day solution that XPs defragger is

I used to use Mydefrag, it's one of the best. The reason I stopped using it is the length at which it can take to defrag. It does a very,very, very good job, but since my HDD has around 420gb filled up, the best type of defrag (forgot the name) took almost a day. Not 12 hours, but 24.
I still install it on other people's computers when doing their tech help and maintenance. Does the job brilliantly. But don't use it anymore since I'd rather not wait for a defrag to take that long again.

For personal use, I've moved onto defraggler. It's been good and does the job well. Individual file defrag is more useful then I thought it would be.

Any of the two above I'd recommend.

I have 2 1TB drives. Had them in RAID0 originally and 6 hours into the MS Defrag I gave up and clicked cancel.

I am using auslogics right now. It's free on download.com. I was using perfectdisk before that, on an old license I bought, but I switched when it had some issues with Win7 (which I am sure they have since fixed).