There being at least four that you might consider - RPI (now somewhat discredited), CPI, CPIH and wage inflation.
More widely I think you have to answer “what is the state pension for” - as @KarenLot says we have one of the lowest state pensions in Europe. Actually I would find a comparison of pension as compared with how much individuals put into “the pot” over their working lifetime much more informative. If the UK has a low state pension because we have low pension contributions then that might be considered “fair enough”.
Certainly one could (I’m not going to though) make the argument that the state pension should only be a safety net and that people ought to make their own provision for their retirement - that would be a pretty “Tory” thing to do.
Assuming you don’t want to take it that far then you probably want to preserve its purchasing power over time. Also, since it is a “wage replacement” you might well argue that it should reflect improvements in average wages over inflation - if the economy is doing well and real terms wages are rising why should pensioners be left behind?
You might also consider - if you were socially minded - that, as it is one of the lowest in Europe, increasing it over time is no bad thing.
So the triple lock is actually quite clever. It stops the value of the pension eroding over time and if inflation is extremely low or real world wages growing there is a gradual uplift in the real world value of the pension.
How is the UK pension doing against these goals? Badly, especially since 2016 - I put together some data:
Yes there has been an increase in the absolute amount of the pension (blue and red lines) but in real terms , measured against CPI/RPI or wages it’s basically flat or falling.
The old “Basic State Pension” is worth about 8% more than it was in 2000, but has fallen by 7% compared with 2016, the “New State Pension” has also fallen the same amount in real terms since 2016, mainly in the last two years as inflation has been so high.
FWIW I don’t think the triple lock was introduced to gain votes, if it was then I doubt that it will be working, I also don’t think they like it all that much as evidenced by the fact that they seem quite ready to claim “exceptional circumstances” mean they don’t have to abide by it. I certainly don’t think they want a scheme which slowly increases the real terms value of the state pension - because they really, really don’t care about anyone poor enough to be totally reliant on the state pension.