For longest life, you can pick either of two answers.
M class red dwarfs are the longest-living fusion stars. These are small stars (less than half the Sun's mass) with low enough pressure/temperature at their cores that they are burning *very* slowly. Lifetimes will exceed 50 billion years, which should be plenty of time for the Universe to completely disperse before these things fizzle out.
L, T, and Y-class brown dwarfs do not fuse hydrogen (except a little deuterium early in their lives) and are therefore only warm due to residual heat of contraction. These will live forever unless they collide with something or are blown to smithereens by a local shock waye from a supernova.
For your other question, high absolute mag corresponds to large and hot-burning stars. Their mass is high, but the resulting core pressure/temperature mean they burn their fuel at a prodigious rate, and will quickly lose the ability to fuse and will collapse. A twenty-solar-mass star will last only 5 million years before it burns everything to iron and goes supernova.