If motion sickness is a consideration
As a child suffering from motion sickness I would always try to sit between or above the wheels. In modern buses you do not see the wheels but the area between the wheels is still the area with the least swaying motions.
If you want forward view, the front seat on the other side as the driver is great. In some buses there will be several to chose from as they are on different levels.
An other advantage of the front row seat on many coaches (as mentioned by @nsn in his answer) is that there is no row further forward with seats that recline into your face.
A disadvantage of the front seats, specially in countries with many road accidents, is that you are more likely to suffer, (or die,) if the bus hits something head on.
My last multiple day 'bus' tour was in a 20 seater that was filled with the maximum number of passengers so the 'guide' seat next to the driver needed to be used. I loved it though it had less leg room and no place to store a carry-on bag. The views were amazing.
Traveling with a group
But for those who like to hang out together while in a bus, the back seat still offers the best options.
Some buses (the kind that have been used as city bus in the early part of their life) have sets of seats where two face backward, making a group of four for friends, without having to be in the back of the bus.
Window or aisle seat
View to the left or the right does make a difference but unless you know where the best views will be, it will be a guess.
I prefer aisle seats, as with those you have pretty good views one way and acceptable ones the other, while window seats give good views one direction and almost nothing the other way.
Other reasons to chose window or aisle is the space you get and the fact that others will pass.
Those that need more than average space, for reasons of build, size or because being stuck in a small space is uncomfortable, the aisle seat is the prefered one.
When needing to get up several times, bathroom or just stretching your legs, you will find the aisle seat the easier one, specially when sitting next to a stranger or when your traveling companion is likely to sleep most of the time.
When sleeping, or trying to, in an overnight bus, the window seat allows for more positions to relax, including leaning against the window.
Sitting in the window seat you will less likely be disturbed by people walking past.
And when in a bus/coach where not everybody is to be trusted, the window seat also gives less access to your things for people walking past.
The trade off is that you have less access to and less control over the items stored overhead.
Left of right of the bus
If you want the best views out of the side windows of a bus, check where the best views will be.
Other considerations are where the sun will be, as sitting on the sunny side on a hot day and looking into the glare for most of the day will be less pleasant. This is made less important if the bus has coating on the windows and if it has good airconditioning.
Not all buses are build the same, exceptions happen
If there is the option to reserve a seat before comming into the bus, remember not all buses are the same.
In some cases the aisle seat behind the driver is a good choice, if you can see out to the front of the bus. On many buses the view to the front is over the driver and the space for the feet is nice.
In some buses you have no forward view at all and less space for your feet.
My grandmother had a knee that she could not bend. She would request the seat next to the emergency exit as that would have enough space for her to sit. But in some cases the seat was not suitable and she had to change the request while getting into the bus the first time. She traveled bus tours for many years, with a company that made the people move to the next seat after every stop, so that all people got the better and the worse seats. She did get freedom from that moving, but it would not always get her a good seat.
Double decker buses, up or down
If you want steady, not too much sway, top level of double decker bus is not good — the movements of the bus will be 3 to 10 times as strong on the higher floor.
Someone suffering from motion sickness should check whether they feel alright on the top floor of a bus. Not all sufferers will need to avoid top floors, and those who suffer in some buses might feel alright in others.
The views are better, specially those from the front seats. And often there are two sets of front seats available.
Extra tips for those who suffer from motion sickness
Watch what you are eating and drinking before and during the trip.
Avoid foods that make you want to burp, check whether you react badly on fatty foods, foods that make you fart (also on the day before a travel) and other foods that might upset the system.
(I would not believe it, but some people claim that drinking carbonated drinks to make you burp helps in not getting sick. Worth trying out if you suffer badly.)
There might be foodstuffs that help you fight attacks, like mints or boiled sweets you suck on. Talk with people in your family if there are more that suffer the sickness as well, as there are bound to be stories of what helps and what to avoid.
If there is a vent for the airconditioning you can aim, have it at least at its lowest level and aimed just in front of your face, unless you want it fuller open and aimed differently of course.
When there are only fixed vents, you may want to chose your seat to have one near, as long as the air coming out smells nice.
Also, the seats behind and below the escape hatch when opened will have a better air quality than the average for the bus.
For those of us who suffer badly, frequently or violently, warn the driver and ask for him to open the door, when safe to do so, on request.
Sometimes a flow of fresh air will calm down the urge, in other times being able to aim out of the bus will be much easier than using a bag.
And always carry a suitable container to accept more than one time of being sick, or a series of 'just big enough' ones.
Best if they are easy to close safely and sturdy enough to withstand the liquid. Paper bags are OK as long as you can dispose of them soon after use.
Strong plastic bags will keep much longer. But smells coming out will not help you nor other people around, so close and keep closed unless needed.
Most people suffering motion sickness can not read on a bus, some exceptions can read but can not stand being bored. So do test and it may turn that you can read.
Always take something that takes away boredom, like music or a spoken book on headphones, or use your brain to distract you, like dreaming up a story or doing your tables of multiplication.