The Volta a Portugal uses mostly the same terrain because a) there is more interesting terrain for racing in the north of the country, b) there are more cycling fans in the north of the country, and c) they now only have 11 days instead of 15 to use the country's variety. I love the race, but the decline in the number of national teams from the 10 or so in 2002-3 to just 5 today is part of that, as is the level of difficulty - mountains like that, heat like that, in the middle of August, it's great preparation for youngsters for GT riding, but it does put off a lot of big teams from entering, as does the very slim possibilities for success because of the strength and focus of the Portuguese teams on the race as the obvious peak of their calendar. But I love the race and wish it got more attention, definitely.
Burgos is simultaneous and is also a favourite. It's a tricky race, it finishes on one of my absolute favourite MTFs, and they do an expert job of making all the stages meaningful and encouraging interesting racing, but still allowing people to stay close enough to make Lagunas de Neila explosive. Great race.
The Giro di Lombardia - it's almost the forgotten Monument, being so far removed from all the others. And it's my favourite of them.
Vuelta a Asturias is a good shout - because a lot of the Spanish stars build up with races like Castilla y León, Catalunya and País Vasco for the Ardennes, its late-April early-May slot means it often has a fairly weak field; it also invites a number of South American and Portuguese teams for whom it is a rare chance outside their own country to showcase themselves, which gives it variety. The LA-MSS debacle in 2008 has hurt it somewhat though.
The Tour de Romandie - at least when it has a pretty good parcours. You often see some talented youngsters and a thimbleful of top names holding form from the Ardennes or warming up for the Giro. And the scenery in Switzerland is some of the most beautiful you'll ever see. Places like Vallorbe are views to die for.
Settimana Internazionale di Coppi e Bartali - similar route every year but always features the cream of the Italian Continental crop trying to lay the first markers down for the Giro after San Remo.