I've spent a few months in early 2017 looking for my next game to speedrun. While most of the games I tried were untouched at the time, I did learn a lot of things from this experimental process.
A lot of people generally say "run a game you like" when suggesting games to speedrun. I like a lot of games, but there's some I wouldn't ever try to seriously speedrun, for different reasons, so I don't think the answer is as simple as it sounds. Based on my experiences, these are factors that have influenced my decision to continue/quit speedrunning some games.
There are two main types of factors to consider: Elements within the game itself and your own motivation for speedrunning/what you want to get out of it.
How often do you get instances where you can look away from the screen and not lose any time? How long are they generally? If these instances do exist, can they be turned off or skipped entirely?
Generally, the less downtime there is in the game, the better. Exceptions might be for longer runs where an unskippable cutscene could mean a quick bathroom break.
Examples of downtime include:
Randomness in a speedrun is not necessarily a bad thing. The main issue is when the run relies solely on a random event to continue or when the run can lose a lot of time due to a random event occurring and there's nothing that can be done to prevent it.
I'm perfectly okay with randomness as long as it's something that can be either controlled or adapted to without much time loss. It's mostly a question of how long are you willing to put up with potential time losses you can't control.
RNG manipulation is usually a set of motions done to mitigate this factor for games that are heavily random by nature, and should be checked for if randomness really does affect the speedrun a lot.
Examples of bad RNG include:
Good RNG is when bosses have a random set of moves they use, but all of them give you equal windows of opportunity to attack the boss.
How fast is the game's pace? Does one chapter took over 2 hours to clear while the others only take 1? Does your character move slowly throughout the entire game, or can they move a lot faster later on?
Generally, if you play a game where you have to unlock powerups and more game options as you progress, the speed at which you progress through stages and levels should increase as well.
This applies to most games, as they usually introduce features that will improve character speed or battle strength as you progress.
How varied are the tricks used throughout the game? How many options for approaching different scenarios do you have at your disposal?
A speedrun that relies on one main trick to go fast gets old after a while. The more options you utilize to go fast, the better. It also feels less repetitive when you're doing runs.
How long is the game in a speedrun setting? What are your limits as to how long you can continuously play a game in one sitting with minimal breaks?
Fairly self-explanatory. I suggest starting with short runs (less than 90 minutes) first and working your way up once you get used to doing runs.
How easy to grasp are the game mechanics? How easy are the tricks to pull off? Does some of the speedrun tech require external programs or a thorough memorization of very precise steps? Are the steps complex and intricate enough that you'll need to reference notes the first few attempts? Are there any very precise tricks that only have a 0.1 second window to pull off? Are there guides and resources out there, or you do you have to learn from watching videos of the best times?
Most games usually have a set of general speed tricks that get applied in specific scenarios, and there's not much else besides that. For other games, there are sometimes tricks or glitches that only apply to one specific room, and there are different variations of this trick depending on the area you're in. Also consider how you're supposed to find this information to begin with.
How interested are people in the speedrun of this game? Are others running it? Are these people generally supportive of newer runners? Is the game currently active? If there are any active runners, will any of them take some time to help you learn?
This will vary depending on your game options. Large communities for popular games are usually skewed towards the more popular/best runners. Some of the best communities I've come across focus on a general interest, rather than a specific game (NES community is a great example). Obviously, if you want to route a new game from scratch, there will be no community, and you'll have to deal with the task of forming one.
Has the game been explored by others? Is there room for improvement? Does the game seem like there's a lot of glitches or skips waiting to be found? Does the game look like it could get in major marathons in the future?
If you're more interested in glitch hunting or routing, then this is what you consider when picking a game. If your goal is to appear in a big live marathon, then consider all the above factors when you decide the game is a good fit for you.
Do you want a world record? How fierce is the competition? Are you prepared to play that game for hundreds and hundreds of hours to reach the top?
Related to the state of the game's community. Generally, the less competitive the game is, the smaller the active community is. The larger and more competitive the game is, the harder it will be to get top times. Being in the top 5 for such a game will usually bring in more viewers, but it's more likely that a large portion of your time will be dedicated to that one game to maintain it, so there's a bit of a trade-off there. Expect to play your chosen game for several months if you're in it for competition.
Did you find the game fun casually? Can you see yourself playing the game over and over?
This will depend on your personal preferences. Some people speedrun games they enjoy, others speedrun games they hate casually but like as a speedrun. Overall, the experience should be fun for somebody, whether it be the runner or viewers.
I'm mainly interested in discovering new things and finding out a game's possibilities. Competition does not really interest me. If a game is new and popular, it's harder for me to make an impact and discover new things. I tend to focus on less mainstream games for this reason.
My main speedgame, Disgaea 5, is a game that I was already well-versed in, so learning curve was minimal. The game runs very fast, has fast loading, completely skippable cutscenes, and a lot of mechanics that make for an interesting routing process. The best time before I come into the picture was around 2:15:00, making it relatively short and manageable. RNG, while present, is of little consequence in the run.
All these factors have led me to stick with the game for as long as I have.