My most-baked cake in a bundt is this Orange Olive Oil Cake. I cut the sugar back significantly (1 to 1-1/4 cup instead of 1-3/4), rub the zest into the sugar, and add cointreau. (The recipe as revised – lower temp, less baking soda – does not sink.)
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However for your purposes, her Orange-Ricotta Pound Cake might be even better – it has a wonderful, rich crumb, and if I had ricotta in my fridge at all times I would make it more often. You can substitute cream cheese for the ricotta for an even finer, dense crumb. It is reminiscent of a creamsicle if you add some vanilla along with the orange.
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On the lemon front, we have two family favorites. This lemon buttermilk pound cake is on repeat. I now bake it as cupcakes for portioning, but it’s originally a bundt. (No fat-free anything – full fat buttermilk or greek yogurt thinned slightly)
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And this cream cheese pound cake has the fine, dense crumb I mentioned earlier. I always make it a lemon cake with the additions from the previous one.
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If you want something a bit different, this Gulab Jamun Cake is essentially a ricotta pound cake with Indian flavors – cardamom, saffron, rosewater.
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I also adapted this Yossi Arefi recipe for the same flavors (no berries) and baked it in a bundt, and it was wonderful.
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ETA: You mentioned grapefruit and white chocolate. There’s a grapefruit white chocolate cake in the first Yossi Arefi book that everyone else loved but was a big fail for me. The orange olive oil or ricotta cakes from Alexandra Cooks can be easily transformed to grapefruit.
