Objectively observed currenly: 28mm GW followed by 20mm WWII.
Personally: started with 1/300/6mm (Battletech) and 1/72 (20mm – various plastic types mostly ancients for DBA and a handful WWII). Back then started to collect some Spacelords 28mm but not many and have given them away and most of the ancients because of lack of players. Kept a few of WWII. Just in case.
Played 6mm sci-fi for the following decades. For me the best for battlegroup type of games (I don´t really can´t get rid of DSII….) and space combat (FT and my favourite Silent Death)
First left my comfort zone last year starting a 28mm sci-fi project. I want to keep it small and with not many figures (Space Trader type of game). Decision for 28mm was because the range of minis for my purposes was greater than in 15mm. My choice was on an emotional basis. I want to keep that small because of the prices. Currrently on hiatus due to:
Started 15mm sword&sorcery this summer and left completely my comfort zone. Fascinating scale and available ranges for my purposes – in fact currently I´ve got only the wish for two certain miniatures (yes, two figures) I haven´t found yet to be completely happy.
And, gladly, there´s even more coming touching spots in me I didn´t know I have. There´s still much
"In strange grammar this one writes" - Master Yoda