There are lots of options.  I favor the OEM tank bag design.  It's practical and SAFE.  Many after market bags are wider than the fuel tank on the typical old airhead.  Due to that design, the bag can contact the kill switch on a tight turn.  An unexpected, sudden loss of power can put the rider on the ground in an instant.  That can be both painful and expensive. 

‘78 R100 / Hannigan here. My long time favorite has been the very period appropriate Vetter tank bags - but they are hard to come by. The rear of the bag is anchored to a strap that goes around the nose of the seat. The front anchors to two snap posts mounted in the fairing. It works on the airhead w/fairing very well. They came in 18L, 24L, and 36L sizes. I have all 3 sizes, and use the 24L the most by far. 

I've had a Wolfman Luggage tank bag for over 20 years and 60K miles.  Love it!  I can store a damp sponge and dry rag in the right side pocket that is easy to access while riding to clean my helmet face shield.  Looks like they don't make the model I have with the side pockets, but I still wouldn't hesitate to buy any of their stuff.

https://wolfmanluggage.com/

For 20+ years, I've used an old Cortech series TourMaster, with magnetic mount. It has a zip on/off top section, the bottom section is expandable by zipper, and both sections have a map pocket. I have a GPS, so seldom use the map pocket for maps, but put things I don't wanna dig for in there, i.e. notes, ink pen, electrolyte drink mix packets, lens cleaners, eye drops, etc.. I usually just run with the bottom section and expand it as needed, and add the top section when going on a trip. Together, they carry about as much as one saddlebag on my 95 R100RT. The magnetic base is super convenient; when gassing up, I just lay the bag on the right side of the handlebar; when finished, just drop it back in place on the tank. Keeping the pad bottom clean insures no paint scratches. It has external pouches on sides and ends that are very handy. 

Only gripe, it is not water proof, so if expecting rain, need to put anything I wanna keep dry in plastic. I carry 1-2 kitchen size garbage bags for that purpose, tucked in one of the external pouches. .  

I'm using a Mosko Moto Nomad now Nomax on 3 bikes with a 4th to come. I really like how it's set up with organizing pockets etc. I use it on my KTM 640 Adventure, my R1100S and my '82 R65 LS. I plan to put it on my KTM 525 when that's on the road too. 

https://moskomoto.com/collections/motorcycle-luggage/products/nomax-tank-bag

Why don't you guys like magnetic tank bags?  Looking for a reason.  I have a Tourmaster smaller tank bag

Bob Thomas, I like magnetic tank bags, and would not bother with anything else. Different strokes...

@bob-thomas I have a magnetic tank bag.  I used it for a few months on my R80.  Investigation showed a lot of small scratches where the magnets contact the tank.  YIKES!  folded the magnets in and run it with the straps.

I use a Dowco Fastrax magnetic bag on my RS. It's plenty roomy and has mesh pockets inside to organize smaller items. Also the interior is fluorescent green which makes it easier to find things. If you remove a magnetic bag from a bike, make sure nothing has adhered to the magnet tabs that might scratch the tank. As stated earlier, it's a personal choice.

I use a Cortech magnetic on my R80 G/S. Great little bag, easy on & off. Never have seen a scratch or blemish in several year's use.

I have used the BMW bag that was available but not anymore.   Man, it could pack a lot of stuff in it. St.