At minimum you should probably run 96 timid speed on mega lati to outspeed max speed base 90s (even though they're not really a thing, but just a benchmark to be safe), but I prefer to run 136 timid to outspeed max speed base 95s (rayquaza and especially kyurem-w, both of which you can ohko with draco meteor) or even 176 timid to outspeed everything up to and including timid palkia.
Max HP for bulk and normally I put the rest of the EVs in defense to help take physical attacks. I don't invest in special defence since even uninvested it easily avoids the 2hko from groundceus ice beam after rocks, whilst stronger ice beams from pogre and mm2y require full sdef investment to avoid the 2hko, giving up mlati's speed tier in order to tank attacks from threats that it can't beat anyway.