My pick (for some teams it was really easy, for some it was a more tough choice):
1. KV Oostende (obvious)
2. Club Brugge (for derby reasons)
3. Anderlecht (always interesting to play them, regardless of your opinion on them)
4. Cercle Brugge (for derby reasons)
5. KV Kortrijk (very tense derby for KVO with a lot of rivalry involved)
6. RC Gent Zeehaven (my favourite team from my native city... AA Gent doesn't attract me)
7. Standard Liege
8. Sporting Charleroi
9. OH Leuven
10. KAS Eupen (the first even German-speaking team in the top division)
11. Germinal Beerschot
12. Union St Gilles (most beautiful stadium in Belgium IMO)
13. RFC Liege
14. Rupel-Boom (for the friendship ties with KVO and their nice stadium)
15. Turkse Rangers (for very obvious reasons for me)
16. RE Virton (very nice away trip, idyllic little village)
17. Eendracht Aalst (not exactly sympathising with them, but they add colour to the league)
18. RAEC Mons
I realise some choices will be seen weird, but that is why I put some explanation behind them.
I left out clubs like KV Mechelen, RC Genk and the Waasland clubs, because they simply leave me cold and emotionless. I replaced them with a few lower league teams that I really have sympathies for.
I think my league is a sort of in-between of strong teams that fill the stadiums (the top-3 teams + other somewhat attractive top divisionists) and the other half are less obvious choices but just clubs I personally like.
I could have also included FC Kosova and Maccabi Antwerp where I have very strong sympathies for, but they would be committing suicide in the top division. The only provinciale team I included are Turkse Rangers because obviously I have a strong emotional connection with Turkey, and because in the Genk area this club could really grow big if they would be in a higher league.
I think only Turkse Rangers, Virton and Eupen may seem weird to some people, but like I said, I followed my personal sympathies here and not just rationalism and logics. The other 15 make sense to anyone I think, no?
OK; name your ideal 18