The rear derailleur on my road bike is out of alignment. When I shift to the smallest (highest) gear, it is visibly out of alignment and has difficulty shifting up. Is the Park Tool Dag-2.2 worth it? I have 2 other derailleur bikes and my son has one also, not loving the price though. I have also adjusted the barrel adjustment with little luck.

Dave

You don't give any info on why or how this may have happened. Has the bike been left laying on its drive side? Has the bike been dropped on its drive side? Has a stick jumped into the rear wheel? etc etc etc If you want good suggestions give good information.
You don't give any info on what kind of frame or rear derailleur so it is a bit hard to offer useful suggestions but the hanger is the first to check.
First thing to do is to check the rear derailleur hanger alignment.
If it is aligned then next is to check the derailleur itself. You want to make sure the derailleur itself has not been bent or the bushings, springs, etc. are not worn to the point where it no longer holds the derailleur in the correct position.
Next is to check the limit screws...honestly unless someone has been screwing around with them they are correctly set or have been incorrectly set for a long time. If the hanger is bent or the derailleur is bent it may look like the limit screws are out but that is wrong and why you always start with the hanger alignment then derailleur.
If the above are all ok then check the derailleur cable for stretch. shift into the smallest rear cog and lightly tug on the cable. If it is loose, floppy, it is likely stretched and needs retensioning to bring it to the correct tension to shift correctly.
There are Youtube videos showing how to adjust a derailleur and it is far easier to watch than try by reading how.

Lastly, if you don't know what you're doing and just start adjusting you are likely to waste a lot of time and wind up nowhere but with a still mis adjusted derailleur...bring it to a shop.
You also need the correct tool for the hanger...you can eyeball it but generally it only works kinda sorta when it is badly misaligned and you have done it enough times to know what you are seeing...the tool is not cheap and you can likely get it checked, aligned and the derailleur adjusted at a shop for far less...but it is a good tool to have if you like to work on bikes and work on a lot of them.
I'll say it again...if you don't give enough info you will never get good advice and/or suggestions.
Good luck

When I have had problems and fiddling didn't fix it, my problem has been the hanger alignment.

As long as you can get access to a second rear wheel (other bike, friend's bike) this has worked very well for me and avoided the purchase of an expensive tool:

The park is a sweet tool but if you are handy check this out and it might cost you $10 to $15 in parts: . My homemade is only slightly more complicated with a nut welded to the end of the bar for the bolt and it has done dozens of perfectly acceptable RD hanger alignments.

Yes, if you're planning on doing your own maintenance for years to come it will pay for itself a few times over compared to taking it to a shop. Here's a good article that reviews a few other tools so you know what you're getting with the Park in comparison. The best derailleur hanger alignment tool: 9 tested - CyclingTips I've got one of the Park's Older verstions of the Dag 2.2, had it for over 20 years and maybe used 50 times on various bikes. It's bombproof and a little clunkier to use than the other higher $$ tools but no complaints for my uses and Park is very good in supplying parts and support.

I don't know what happened to the bike. It is a Scattante road bike, was never crashed and has always been meticulously cared for. Was working on it tonight and both the hanger and derailleur were loose. Tightened them up, and now it shifts really well again.

Dave

I bought an inexpensive one from an Ebay seller a couple of years ago. For casual home use - say a few times a year- this isn't a really complicated tool and the cheap ones work well. For shop use I don't think it would be ergonomically efficient or durable enough. But certainly accurate for the precision that is needed.

The one I have - or functionally identical - is marketed under a variety of names in a wide variety of online sources. It's called the "Lifeline Pro Gear Hanger Alignment Tool – Best value" in this excellent article. The best derailleur hanger alignment tool: 9 tested - CyclingTips I'm pretty sure I found one for closer to $30 under a different name, from an Ebay seller, and have used it several times to good effect. If you do a simple web or EBay search for "derailleur alignment tool" you'll see many different ones that are about the same.

99% of the time its the hanger. Get that right and the barrel adjuster is a 10 second affair.
I think a tool like this is indispensable.
And like so many pricey tools, they're so good you ask yourself why you dithered.